LOS ANGELES (AP) _ Gasoline prices remained virtually unchanged from two weeks ago, as retailers battled it out to win over motorists with lower prices, an industry analyst said Sunday.

''Street competition is so hot, in many cases a wholesale increase cannot get passed through to the consumer,'' said Trilby Lundberg, publisher of a twice-monthly nationwide survey of prices at 15,000 gas stations.

''There's a persistent conception that prices bump up when there's a holiday,'' she said. ''It is supply and demand, not the calendar, that defines gasoline prices, and this New Year's season is characterized by a holding pattern on average.''

The Dec. 19 survey found the average price for all grades of gasoline, plus tax, was 84.9 cents. The average price Dec. 5 was 84.8 cents.

The survey found regular unleaded gasoline selling for an average 76.6 cents per gallon at the self-service pump, regular leaded selling for 71.91 cents and premium unleaded averaging 91.16 cents.

Ms. Lundberg noted a slight drop in the price of self-service premium unleaded. Two weeks ago, it averaged 91.44 cents per gallon. It was the only grade of gasoline that dropped in price since the Dec. 5 survey.

At full-service pumps, a gallon of regular unleaded averaged $1.07.55, regular leaded sold for $1.01.97 and premium unleaded cost $1.17.

Last December, a gallon of gas averaged $1.20. Ms. Lundberg said American motorists have saved almost $26 billion as a result of lower prices brought about by the world oil glut.

She said prices could go up 6 1/2 cents per gallon if the Organization of Petroleum Exporting Countries carried through a plan formulated over the weekend to raise oil prices to about $18 a barrel and cut production. Oil prices on the open market now range from about $13 to $16 a barrel.

But Ms. Lundberg said there was no way to predict if such an oil price increases would affect prices at the gas pump.

''Even a crude oil price increase of a few dollars per barrel is not enough to pull prices back up to $1.20 anytime soon, because of competitive pressures,'' she said.