A Houston firm was fined $750,000 yesterday for its role in a scheme to pay more than $500,000 in bribes to officials of the Mexican national oil company. The company agreed to cooperate with a continuing government investigation of bribery of Mexican officials by U.S. oil field equipment companies.

As part of a plea agreement between the company and the Justice Department, Ruston Gas Turbines pleaded guilty to one count of violating the Foreign Corrupt Practices Act.

The government charged that Ruston paid money to suppliers that trickled down to two high ranking officials of Petroleos Mexicanos (Pemex) who aided the firms in obtaining contracts.

The government said that Ruston paid the C.E. Miller Co. for gas compression systems process equipment, that part of the money was then paid to Crawford Enterprises Inc. and that some of that money was taken by Crawford employes and agents.

A Crawford agent then paid 5 percent of the contract price to the Mexican officials, the government alleged. In the case of the specific 1978 contract cited, the percentage amounted to $317,382. The Mexican officials also received a $200,000 sweetener, the government said.

The government said that Crawford's president had set up a bribery scheme in 1977 involving the two Mexican officials, Jesus Chavarria, sub director in charge of production, and Ignacio DeLeon, sub director in charge of purchasing.

Crawford has been identified as a target of the investigation but has not been charged with any offense.