WASHINGTON (AP) _ Six publishers of scientific and technical journals filed suit Monday, alleging that Texaco Inc., the nation's third-largest oil company, has violated their copyrights by ''large-scale, covert, unauthorized photocopying.''

They alleged, in the lawsuit filed in federal district court in New York, that Texaco had ''previously assured publishers it would correct such conduct and practices'' but has continued the copyright violations.

Bob Hoxie, a Texaco spokesman, said his company would withhold comment until it has seen the suit. The case was assigned to U.S. District Judge Pierre N. Leval.

Plaintiffs are the American Geophysical Union, Washington, D.C.; Elsevier Science Publishing Co. Inc., New York; Pergamon Press Ltd., Oxford, England; Springer-Verlag GmbH, Berlin, West Germany; John Wiley & Sons, New York; and Wiley Heyden Ltd., Chichester, England.

The suit, which did not ask for a specified sum, sought actual damages, an accounting and an order halting the practice.

Since 1978, the suit said, Texaco has photocopied scientific and technical journals without authorization ''for all but token quantities,'' and has not paid the required fees. Therefore, it said, Texaco ''has misappropriated the fruits and values'' of the publishers.

The journals ''result from substantial investments of time, money and other efforts'' and the publishers rely on revenues from their sale, distribution and licensing, it said.

Texaco is the country's fifth-largest industrial corporation in terms of sales. The suit said that its 1983 research and development expenses were reported to be $140 million.