A D.C. Superior Court judge declared a mistrial yesterday in the case of a private security guard who was charged with murder after it was disclosed that a jury member apparently had consulted reference books for definitions of various criminal offenses.

The guard, Ronald King, 21, of 662 Acker St. NE, was charged with second-degree murder in the shooting death of a youth who was a customer at a Hot Shoppes restaurant on V Street NW, where King worked as a security guard. The victim, Kenneth Charles 18, was shot once in the chest during a scuffle with King and another youth at the restaurant on Dec. 19, 1977.

Jury foreman Sheila Williams said the jury was stalmated at a 9-to-3 vote to acquit King of the murder charge, as well as the lesser offenses of voluntary and involuntary manslaughter, when the mistrial was declared.

Judge John Garrett Penn received a note from Williams at about noon yesterday in which she said "the jury has experienced a real problem." She explained that one male juror, whom she did not identify, had "done outside research" on some of the charges. Williams said in her note that the juror had "gathered definitions for different words from his law books . . . "

The jury had deliberated for about three days when the mistrial was declared. King's trial began on Oct. 2.

On Wednesday, the jury returned to Penn's courtroom and said they had reached a verdict acquitting King of all three charges they considered. However, when Assistant U.S. Attorney Carol Bruce asked for a poll of the jurors, it was revealed that some jurors had voted to convict King on one of the two manslaughter charges.

Penn then told the jury that they would have to resume their deliberations, and later instructed them that they must reach a unanimous verdict. Foreman Williams said yesterday that the jury members thought that they could reach a valid verdict with a vote of the majority. Their announcement on a verdict was based on a 7-to-5 vote for acquittal on all counts, she said.

King's lawyer, O.B. Parker, said his client had been employed by the Pittman Detective Agency for about four months at the time of the shooting. King contended that he drew his gun in self defense and that the weapon then discharged accidentally.