MIAMI - Troubled running back Ricky Williams is appealing a positive drug test that could force the Miami Dolphins star to sit out the 2006 season, a person familiar with the case said Monday.

If the positive test result is upheld, it would be Williams’ fourth violation of the NFL’s substance-abuse policy, meaning at least a one-year suspension.

Previous positive tests were for marijuana, which Williams acknowledged using. But the latest test involved a drug other than marijuana, said the person, who spoke on condition of anonymity because league policy is to keep test results confidential.

The Miami Herald reported Monday that two sources said Williams failed a test for a substance not publicly known. His latest positive test was first reported Sunday by Denver television station KDVR.

A ruling on the appeal isn’t expected until April. Williams, the 1998 Heisman Trophy winner and 2002 NFL rushing champion, is in India studying yoga and holistic medicine.

“Ricky Williams has been in outstanding behavioral mode for the past six months. No one should jump to any conclusions,” said Williams’ agent, Leigh Steinberg.

The Dolphins and the NFL declined to comment, citing the private nature of the league’s drug program.

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Williams retired and sat out the 2004 season. He owes the Dolphins $8.6 million for breaching his contract, and motivated at least partly by the need for a paycheck, he returned last year to play for new Miami coach Nick Saban.

Before being allowed to return, Williams served a four-game suspension at the start of the season for his third violation of the drug program.

Despite the long layoff and a limited workload, Williams ran for 743 yards and averaged 4.4 per carry while sharing playing time with rookie Ronnie Brown. Williams gained 280 yards in the final two games when Brown was hurt.

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Saban and the Dolphins praised Williams’ attitude, work ethic and leadership. Dispelling his reputation as an aloof, selfish pothead, Williams became such a model of good behavior that he was chosen co-winner of the local media’s annual postseason Good Guy Award, given to the player most cooperative with reporters.

Williams laughingly described himself as weird and compared his career to a roller coaster. Since returning last season, the NFL has required him to undergo drug tests up to 10 times a month.