Two Northern Virginia paving companies and the former head of one of the firms pleaded guilty in Richmond yesterday to federal bid-rigging conspiracy charges, the result of a 10-month investigation into corruption among Virginia highway contractors.

The defendants joined four other corporations and individuals, also indicted this spring by a grand jury in Alexandria, in acknowledging corruption in the letting of lucrative contracts by the Virginia highway department, which maintains most of the roads in the Washington suburbs. Trial for all seven defendants had been scheduled for July 6.

Appearing yesterday before District Judge Richard L. Williams were Dominion Paving Inc. of Leesburg, a subsidiary of William A. Hazel Inc.; General Paving Corp. of Manassas, and Jack A. Mansergh of Hartly, Del., retired president of General.

Under a plea agreement with the Justice Department's antitrust division, Dominion and General each will pay $250,000 fines. Separate mail fraud counts against the two companies were dismissed. Justice lawyer Hays Gorey Jr. said the department also has agreed not to use grand jury testimony gathered in connection with Dominion in any possible future prosecution against the Hazel firm or its affiliates.

Gorey refused to say whether William A. Hazel, a Fairfax County businessman and head of the company that bears his name, or four of his companies that were named in the plea agreement are targets of the ongoing investigation.

Mansergh yesterday pleaded guilty to a single conspiracy count of the indictment alleging collusion on state contracts back to 1977. In addition, Gorey said, Mansergh has agreed to acknowledge complicity in a separate conspiracy allegation involving a $1.3 million paving contract for work in 1978 on Interstate Rte. 66 in Fauquier and Prince William counties. A mail fraud count against Mansergh was dismissed.

Sentencing for Mansergh, who faces a maximum penalty of six years in prison and $200,000 in fines on the two conspiracy charges, was set by Williams for July 23.