SAN DIEGO (AP) _ A Muse Air DC-9 about to take off with 89 people aboard was evacuated Monday when it filled with smoke, apparently caused by a leak in a hydraulic fluid line, authorities said.

''There was no fire,'' said Muse Air station manager Toby Pratt. ''At this time, it appears some hydraulic fluid leaked into a hot part of an engine, which, in turn, caused the smoke.''

Four passengers aboard Flight 878, which originated at Lindbergh Field and was bound for San Antonio, Texas, via Los Angeles, were evaluated for possible smoke inhalation, but paramedics determined they didn't require treatment, Pratt said.

A preliminary investigation showed the smoke aboard Flight 878 came from a leak in a hydraulic fluid line, said Muse Air station manager Toby Pratt.

The jetliner had been taxiing down the runway about three minutes when a light in the cockpit came on indicating low hydraulic fluid pressure, Pratt said.

The pilot ordered an immediate evacuation of the 84 passengers and five crewmembers, he said.

Passenger Jim Regan of Carlsbad, Calif., said he was sitting in the back of the jet when he noticed the smell and saw smoke.

''First there was an acrid smell that kind of burned the nasal passages. About five or 10 seconds later, smoke came drifting out of all the vents,'' Regan said. ''As soon as that happened, a stewardess ran to the cockpit and pounded on the door to let the pilot know what was happening.''

''There was quite a bit of coughing,'' said another passenger, Roger Willison of Los Angeles. ''I didn't know if it would stop. It just started getting thicker and thicker.''

''People were complaining about it, but everybody's just delighted the plane hadn't gotten into the air,'' Regan said.

The aircraft would be repaired by workers in San Diego and then flown to the airline's West Coast maintenance base in Los Angeles, where workers would try to determine what caused the fluid leak, Pratt said.

Muse Air, which is owned by Southwest Airlines, has been operating out of San Diego since Oct. 27. It has scheduled flights to Texas and Florida from San Diego.