WINDOW ROCK, Ariz. (AP) _ A tribal judge denied a motion today to dismiss charges against the son of suspended Navajo Chairman Peter MacDonald to force him to testify against his father.

Special prosecutors asked Judge Robert Yazzie of tribal District Court on Wednesday to dismiss the single conspiracy count against Peter ''Rocky'' MacDonald Jr. and to compel him to testify.

Yazzie denied the motion without explanation this morning and resumed the trial, which had been temporarily halted while he studied the prosecution request.

The two are on trial on bribery, fraud and conspiracy charges stemming from the tribe's 1987 purchase of the Big Boquillas Ranch for $33.4 million, just hours after it had been sold to a third party for $26.25 million.

The elder MacDonald is charged with single counts of bribery, fraud and conspiracy and 10 ethics violations. The single conspiracy count is the only charge facing his son.

Though he turned down the prosecution today in the motion regarding the younger MacDonald, Yazzie had granted a prosecution motion Wednesday to dismiss election-fraud charges against suspended Vice Chairman Johnny Thompson, a defendant in an upcoming trial. The dismissal of charges is in exchange for Thompson's testimony against MacDonald both in the current Big Boquillas trial and the election-fraud trial set for next month.

MacDonald and Thompson were suspended by the tribal council in February 1989 after a U.S. Senate committee heard testimony in Washington from Rocky MacDonald and others that the elder MacDonald accepted cash payments and other favors from people doing business with the tribe.

The MacDonalds were convicted on Oct. 17 in tribal court of bribery, conspiracy and ethics violations in October. That trial involved allegations that the elder MacDonald took bribes and kickbacks from businessmen operating on the reservation.

The elder MacDonald is also to stand trial Jan. 16 on election fraud charges. Until the charges against Thompson were dropped Wednesday, he had faced more than 60 counts in that case stemming from the tribe's 1986 election.