Al-Zawahri is considered the organizational brain of the terror group, highly skilled at planning and logistics. Analysts said he could set his sights on a spectacular attack and on building up al-Qaida's already robust presence in Yemen to establish his leadership credentials.
His fanaticism and the depth of his hatred for the United States and Israel are likely to define al-Qaida's actions under al-Zawahri's tutelage. In a 2001 treatise that offered a glimpse of his violent thoughts, al-Zawahri set down al-Qaida's strategy: to inflict "as many casualties as possible" on the Americans.
"Pursuing the Americans and Jews is not an impossible task," he wrote. "Killing them is not impossible, whether by a bullet, a knife stab, a bomb or a strike with an iron bar."
Al-Zawahri's hatred of America was also deeply personal: His wife and at least two of their six children were killed in a U.S. airstrike following the 2001 U.S. invasion of Afghanistan after the 9-11 attacks.
The Egyptian-born al-Zawahri had been expected to inherit al-Qaida's leadership, although the delay in announcing his succession led some counterterrorism analysts to speculate about a power struggle following the May 2 killing of bin Laden in a U.S. raid in Pakistan.
"The general command of al-Qaida, after completing consultations, declares Abu Mohammed, Ayman al-Zawahri, God help him, the one leading the group," said a statement attributed to al-Qaida and posted on militant websites, including several known to be affiliated with the group.
It gave no details about the selection process but said the choice of al-Zawahri was the best tribute to the memory of the group's "martyrs."
Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, said the U.S. will pursue the new al-Qaida leader just as it did bin Laden.
"As we did both seek to capture and succeed in killing bin Laden, we certainly will do the same thing with Zawahri," he said at a news conference in Washington.
Al-Zawahri, who turns 60 on Sunday and has a $25 million bounty on his head, takes control of al-Qaida at a time when it is struggling to stay relevant in the face of popular uprisings across the Arab world that are demanding Western-style democracy instead of the pan-Islamic nation sought by Islamists.
Still, the lawlessness gripping Yemen, a poor Arabian Peninsula nation, offers al-Qaida a rare opportunity to gain a strategic foothold in the Arab world, bringing it a step closer to the ability to export its extremist brand of Islam to the region.
"He will send his best fighters and organizers there," said Abdel-Rehim Ali, an Egyptian expert on terrorism and extremist Islamic groups. "Yemen is the closest target and a great start for al-Zawahri to realize his dream of an Islamic emirate."
Al-Qaida militants and their allies in Yemen already have taken advantage of the turmoil there to seize control of towns in the south and strike deals with local garrisons to train with weaponry and live openly.
Al-Zawahri, a trained surgeon who hails from an upper-middle-class Cairo family, lacks the populist appeal of his late boss, throwing into doubt whether he would be able to lure young Muslims, particularly in the West, to join al-Qaida's cause.
In Washington, Defense Secretary Robert Gates said al-Zawahri lacks the "peculiar charisma" of bin Laden and said there is suspicion about him among militants because he is Egyptian.
Still, what he lacks in personal magnetism al-Zawahri makes up for with rock solid ideological conviction and organizational and logistical skills, qualities that may have spared al-Qaida a swift demise following its expulsion from Afghanistan in 2001.
It's not clear how much consensus there was over al-Zawahri's succession, but two U.S. officials said he was not a popular choice. They spoke on condition of anonymity to discuss intelligence matters.
Al-Zawahri and his backers seemed to understand that, so instead of declaring himself bin Laden's successor in his first public video eulogizing the slain al-Qaida leader, al-Zawahri waited for a call by fellow jihadis, said Bruce Riedel, a former CIA officer and al-Qaida expert at the Brookings Institution. The idea was to create the impression of popular support, he said.
U.S. officials said they'll be watching for signs that al-Zawahri is a leader in name only, with affiliates branching out even more on their own.
They noted that communications captured in the attack on bin Laden in Abbottabad, Pakistan, showed al-Qaida's Yemeni branch, al-Qaida in the Arabian Peninsula, argued against bin Laden's idea of spectacular attacks in the U.S. and in favor of smaller operations.
But al-Zawahri's lack of universal acceptance within the organization, analysts said, could give him added incentive to stage a spectacular attack against a prestige target, most likely American, to boost his leadership credentials.
"He must already be planning a big attack to convince the skeptics that he is qualified as a leader," said Khalil al-Anani, an expert on Islamic groups from Durham University in Britain. "He will be under pressure to do that, and quickly."
Al-Zawahri pledged earlier this month to avenge the slaying of bin Laden and to continue the terror network's campaign against the U.S. and other Western interests.
"He was a given leader from the outset. But he doesn't have the same iconic status or personality as bin Laden," said Magnus Ranstorp, a terror analyst at the Royal Swedish Defense College. "He will focus on attacking the West in a big way. To avenge (bin Laden's death), but also to make himself ... even more effective and relevant."
The son of an Egyptian family of doctors and scholars, al-Zawahri's father was a pharmacology professor at Cairo University's medical school and his grandfather was the grand imam of al-Azhar University, Sunni Islam's supreme seat of learning.
He has a long history of radicalism, beginning at age 15 when he founded an underground cell of high school students to oppose the Egyptian government. He later merged his cell with other militants to form Egypt's Islamic Jihad.
Al-Zawahri was arrested in connection with the 1981 assassination of Egyptian President Anwar Sadat and served three years in prison. Many Egyptians remember him as the young man who stuck his head against the bars of the defendants' cage in a Cairo courtroom to answer Western reporters' questions in fluent English.
Upon his release, he headed to Afghanistan in 1984 to fight the Soviets, where he linked up with bin Laden. He later followed the al-Qaida leader to Sudan and then back to Afghanistan, where they found a safe haven under the radical Taliban regime.
Soon after came the 1998 bombings of the U.S. embassies in Africa, followed by the 2000 suicide bombing of the USS Cole off the coast of Yemen, an attack al-Zawahri is believed to have helped mastermind.
Al-Zawahri has worked in the years since to rebuild al-Qaida's leadership on the Afghan-Pakistan border. Al-Qaida has inspired or had a direct hand in attacks in North Africa, Saudi Arabia, Yemen, Pakistan, the 2004 train bombings in Madrid and the 2005 transit bombings in London.
The CIA came close to capturing him in 2003 and killing him in 2004 - both times in Pakistan. In December 2009, they thought they were again close, only to be tricked by a double agent who blew himself up, killing seven CIA employees and wounding six more in Khost, Afghanistan.
The Pakistani Taliban welcomed the appointment of al-Zawahri and vowed to fight alongside the terror group against the U.S. and "other infidel forces" around the world.
"We share the same path with al-Qaida. We are allies," Pakistani Taliban spokesman Ahsanullah Ahsan told The Associated Press by telephone from an undisclosed location.
Al-Zawahri has been in hiding for nearly 10 years and is widely believed to be near the Pakistan-Afghanistan border. He has appeared in dozens of videos and audiotapes in recent years, increasingly becoming the face of al-Qaida as bin Laden kept a lower profile.
AP writers Ishtiaq Mahsud in Dera Ismail Khan, Pakistan, Karl Ritter in Stockholm and Kimberly Dozier in Washington contributed to this report.