But he said the selection of al-Qaeda’s former No. 2 to take bin Laden’s place is a reminder that the terror group is still out there and must be pursued.

“Despite having suffered a huge loss with the killing of bin Laden and a number of others, al-Qaeda seeks to perpetuate itself, seeks to find replacements for those who have been killed and remains committed to the agenda that bin Laden put before them,” Mr. Gates told Pentagon reporters, at what was billed as his last scheduled press conference before retiring at the end of the month.

Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, said al-Zawahri’s move to the top was no surprise, and the new leader will get the same attention from the U.S. that bin Laden did.

“As we did both seek to capture and succeed in killing bin Laden, we certainly will do the same thing with Zawahri,” Adm. Mullen said.

Mr. Gates and Adm. Mullen said they read little into the fact that it took seven weeks for al-Qaeda to name a successor.

Mr. Gates quipped, “It’s probably tough to count votes when you’re in a cave.”