ONDON, Sept. 19 — The police have arrested a 21-year-old man suspected of writing a virus, known as T0rn, that attacked Linux computer systems, a spokesman for Scotland Yard said today. American and British authorities had been investigating the virus for a year.

Officers from Scotland Yard arrested the man on Tuesday at his home in Surbiton, southwest of London.

"Computer equipment was seized from a residential property and a 21-year-old man was arrested under the Computer Misuse Act 1990," the spokesman said, adding that the man was released on bail pending further investigation. The police did not immediately identify the man.

The arrest of virus writers is a rare victory for law enforcement, and the police hailed this case as an important step.

T0rn, which later was modified by a Chinese virus-writing group to form another virus known as Lion, circulated for much of 2001 but did relatively little harm. Computer security experts consider T0rn to be the first Trojan horse-style virus — meaning it masquerades as legitimate software — written expressly to harm Linux users.

T0rn, with a zero, was not as menacing as the Code Red, Sircam and Nimda worms and viruses, which caused hundreds of millions of dollars in damage to corporate computer networks last year. Linux-based software systems account for a small segment of the computing market.

"As far as I recall, it never appeared on any of our top 10 most active virus lists," said Graham Cluley, senior technical consultant at the British anti-virus software maker Sophos.

Nevertheless, the law enforcement and computer security communities regard the arrest as significant.