It takes a computer hacker to catch one.

And if, as federal authorities contend, 31 year-old computer outlaw Kevin Mitnick is the person behind a recent spree of break-ins to dozens of corporate, university and personal computers on the global Internet, his biggest mistake was raising the interest and ire of Tsutomu Shimomura.Shimomura, 30, is a computational physicist with a reputation as a brilliant cyber-sleuth in the tightly knit community of programmers and engineers who defend the country's computer net-works.

Over the years, software security tools that Shimomura has designed have made him a valuable consultant not only to corporations but also to the FBI, the Air Force and the National Security Agency.

And it was Shimomura who raised the alarm in the Internet world after someone used sophisticated hacking techniques on Christmas Day to remotely break into the computers he keeps in his beach cottage near San Diego and steal thousands of his data files.

Almost from the moment Shimomura discovered the intrusion, he made it his business to use his own considerable hacking skills to aid the FBI's inquiry into the crime spree.

He set up stealth monitoring posts, and each night over the past few weeks, Shimomura used software of his own devising to track the intruder, who was prowling around the Internet. The activity usually began around midafternoon, Eastern time, broke off in the early evening, then resumed shortly after midnight and continued through dawn.

Shimomura's monitoring efforts enabled investigators to watch as the intruder commandeered telephone company switching centers, stole computer files from Motorola, Apple Computer and other companies, and copied 20,000 credit-card account numbers from a commercial computer network used by some of the computer world's wealthiest and technically savviest people.

And it was Shimomura who concluded last Saturday that the intruder was probably Mitnick, whose whereabouts had been unknown since November 1992, and that he was operating from a cellular telephone network in Raleigh, N.C.

Sunday morning, Shimomura took a flight from San Jose to Raleigh-Durham International Airport. By 3 a.m. Monday, he had helped local telephone company technicians and federal investigators use cellular-frequency scanners to pinpoint Mitnick's location: a 12-unit apartment building in the northwest Raleigh suburb of Duraleigh Hills.

Over the next 48 hours, as the FBI sent in a surveillance team from Quantico, Va., obtained warrants and prepared for an arrest, cellular telephone technicians from Sprint Corp. monitored the electronic activities of the man they believed to be Mitnick.

On Tuesday evening, the agents had an address - Apartment 202 - and at 8:30 p.m. a federal judge in Raleigh issued the warrant from his home. At 2 a.m. Wednesday, while a cold rain fell in Raleigh, FBI agents knocked on the door of Apartment 202.

It took Mitnick more than five minutes to open it. When he did, he said he was on the phone with his lawyer. But when an agent took the receiver, the line went dead.

Mitnick, 31, was charged with computer fraud, punishable by 20 years in prison, and illegal use of a telephone access device, which carries a maximum 15-year sentence. Both crimes also are punishable by $250,000 fines.

"He was clearly the most wanted computer hacker in the world," said Kent Walker, an assistant U.S. attorney in San Francisco. "He allegedly had access to corporate trade secrets worth billions of dollars. He was a very big threat."

Mitnick already was wanted in California for violating probation on a previous hacking conviction. A hearing was scheduled for Friday.