Twelve jurors gave Paul Skyhorse a very special 33rd birthday present yesterday. They acquitted him of murder. Skyhorse and Richard Mohawk, 28, were found not guilty of the Oct. 10, 1974, murder of cab driver George Aird - a verdict that ended one of the longest, most expensive and most controversial trials in California history.

The high-security courtroom was jammed with supporters of the two American Indian Movement (AIM) orgnizers, and the crowd erupted into applause as the innocent verdicts were read.

The jurors had deliberated for more than 62 hours after a 13-month-long trial.

The case had been in the courts for 3 1/2 years, at a cost to the taxpayers of more than $1.25 million. Skyhorse and Mohawk have been in jail during all of that time.

The defense contended from the outset that the pair had been framed in the death because of their political activities as AIM organizers. The murder occured at what was then known as an AIM camp, 13 miles north of Los Angeles. It was there that cab driver Aird's beaten and stabbed body was found stuffed into a drainpipe. Three persons arrested at the scene and charged with Aird's murder later turned up as prosecution witnesses against Skyhorse and Mohawk in exchange for reduced charges.

Outside the courtroom, chief defense attorney Leonard I. Weinglass told reporters, "It's a victory for native Americans who have not always fared well in these courts. And I think it's a victory for all people in this country who are concerned about justice."

Ventura County Deputy District Attorney Louis J. Samonsky, who prosecuted the case, was bitter about the trial's outcome. "I'm still positive that the defendants are guilty of the murder," Samonsky said, "I'm disappointed for the public's sake whom I represent."

In addition to the murder charges, the case raised several legal issues. For the first time, the California State Bar Association intervened in a trial to seek a change of venue from Ventura County to Los Angeles County on grounds of prejudice because of the actions of the first prosecutor and the original judge.

Both were involved in a skit entitled "The People vs. Tonto," which mocked the trial while it was under way.

The case also prompted a review of the practice of appointing retired judges to handle special cases as was done most notably in this trial and that of Symbionese Liberation Army members William and Emily Haris. That practice has been halted.