WASHINGTON, Feb. 19 - The submarine Jimmy Carter, which joined the Navy's fleet on Saturday, has a special capability, intelligence experts say: it is able to tap undersea cables and eavesdrop on the communications passing through them.

The Navy does not acknowledge that the submarine has this capability. "That's going to be classified in nature," said Kevin Sykes, a Navy spokesman. "You're not going to get anybody to talk to you about that."

But intelligence community watchdogs have little doubt. The previous spy submarine, the Parche, was retired last fall. That would only happen if a new one was on the way, they say.

The $3.2 billion Carter was extensively modified from its basic design, given a hull extension that allows it to house technicians and gear to perform the cable-tapping and other secret missions, experts say. The Carter's hull, at 453 feet, is 100 feet longer than the other two submarines in the Seawolf class.