JOHANNESBURG, South Africa (AP) _ A white policeman testifying at the murder trial of two colleagues said Friday that his unit improperly arrested blacks and tortured them by submerging their heads ''until they talk.''

The constable, Michael Neveling, was testifying for the prosecution against the squad's leader, Warrant Officer Leon De Villiers, 36, and Constable David Goosen, 26.

The defendants are charged with two counts of murder, two of assault and one of attempting to defeat the ends of justice. Conviction on the murder charges could carry the death penalty.

Neveling, in his fourth day of testimony in the eastern Cape Province town of Graaf-Reinet, said the unit had interrogated some blacks while placing plastic bags over their heads or forcing their heads under water.

In previous testimony, Neveling and Sgt. Heinrich Bloementhal have said a black man was stabbed and another fatally shot execution-style during two forays by the 10-man unit into the black section of the town of Craddock on July 26, 1986.

The unit had been sent to Craddock from Port Elizabeth to keep order during a funeral.

Neveling and Bloementhal have said the actions followed heavy drinking and a blood-oath ceremony by the unit members.

Neveling testified Friday that the 18-year-old black who was shot dead had been apprehended because he was wearing a T-shirt with a political logo and because he ran when the officers approached. He said this and other arrests made at the time were improper.

The court heard this exchange between Neveling and B.P. Loots, one of two assessors assisting the judge in the non-jury trial.

Loots: ''What did you hope to achieve by them (the arrests)?''

Neveling: ''You never know until you try. It's pure luck if you get anything.''

Loots: ''And if the person does not want to talk?''

Neveling: ''You make them talk.''

Loots: ''How?''

Neveling: ''As we did. Assaults, plastic bags and so on.''

Loots: ''Is that legal?''

Neveling: ''No.''

Loots: ''You mentioned a water method?''

Neveling: ''Yes, you hold them under until they talk.''

The defendants, who have pleaded innocent to all charges, are the only unit members to be prosecuted.

Neveling and Bloementhal testified earlier that they initially participated in a cover-up, but later decided to tell the truth.

South Africa's apartheid policy establishes, by law and custom, a racially segregated society in which the nation's 26 million blacks have no vote in national affairs. The 5 million whites control the economy and maintain separate districts, schools and health services.