LOS ANGELES, Apr. 20 – The prosecution said it would seek the death penalty as Robert Beausoleil’s trial went into the penalty phase today following his first degree murder conviction in the knife slaying of musician Gary Hinman.

Beausoleil, 22, had contended he didn’t stab Hinman last summer, that hippie-type cult leader Charles Manson killed the 34-year-old musician.

Manson, accused in the killings of actress Sharon Tate and six others, has since been indicted along with two others for Hinman’s death.

The Superior Court jury of seven women and five men who must now recommend death in the gas chamber or life imprisonment returned the verdict against Beausoleil Saturday after two days of deliberation.

After the verdict in the retrial was read, Judge William B. Keene instructed the jurors to begin the penalty phase this morning. Keene also presided over Beausoleil’s first trial which ended In a hung jury last December.

Deputy Dist. Atty. Burton Katz said the verdict “restored my faith in the jury system.”

Katz, who said throughout the trial he would seek the death penalty against Beausoleil, said he had been “concerned whether the jury would be influenced by factors such as the good looks and youthful appearance of the defendant.”

He said he did not feel the county grand jury murder indictments against Manson, Susan Atkins — also accused in the Tate killings — and Bruce Davis, 27, influenced the verdict. Davis has not yet been arrested.

Two prosecution witnesses, Mary Brunner, 26, brought here from Madison. Wis., and Danny DeCarlo, 25, testified that Beausoleil killed Hinman on orders from Manson.

Miss Brunner, a former University of Wisconsin librarian, testified that she saw Beausoleil stab Hinman in the chest 48 hours after she, Beausoleil and Miss Atkins went to the Hinman’s home to try to get $20,000 from him.