One of America's best-known computer hackers pleaded guilty yesterday to charges that he electronically broke into computers used by telephone companies and other corporations.

Mark Abene of New York, known in the hacker community as "Phiber Optik," pleaded guilty in U.S. District Court in Manhattan to one count of conspiracy and one count of unlawful access to computers. Abene is the last of the five young hackers indicted in the case to plead guilty. The group called itself "MOD," short for "Masters of Disaster" or "Masters of Deception," and used a variety of computer techniques to penetrate the systems' security.

Abene, 21, faces a maximum possible prison sentence of 10 years and $500,000 in fines, though the actual sentence is expected to be far less than that, his attorney said yesterday.

The government's MOD investigation, which began in 1989, grew out of complaints of computer tampering by New York Telephone and Southwestern Bell. It was the first probe of computer hackers to use court-authorized wiretaps of phone conversations and data transmissions.

The defendants were charged with gaining electronic access to computer switching systems at such companies as Pacific Bell and US West, as well as systems at credit reporting agencies such as Trans Union and TRW, where they allegedly retrieved more than 100 credit reports. Some members of the group were accused of accepting money for their hacking services, though Abene was not.

Abene had long argued that he had done no damage and that laws against computer hacking unnecessarily restrain inquisitive minds. He decided to plead guilty rather than risk a stiff sentence if the case went to trial. "It's just too many question marks and not enough guarantees," Abene said yesterday.