Christopher Stokes had been a detective and a highly decorated officer for Richmond's police force before he joined the Fairfax County Police in 1967. Two years later, Fairfax honored him as its policeman of the year and the officer seemed to be headed for an equally distinguished career in Northern Virginia.

But, Justice Department lawyers argued yesterday, something went awry with Stokes' plans. What went wrong, they alleged in papers presented to a federal judge in Alexandria, was that Stokes was black -- the first officer on the Fairfax force -- and he ran into what the lawyers charged was a pattern of systematic racial discrimination in the county government.

In Stokes' case, the lawyers said he was denied a permanent investigator's position despite his "numerous requests" for such a job. When he applied for promotion to the rank of corporal, the attorneys alleged his supervisors were directed to give him a personnel rating that would disqualify him.

The Stokes case "provides an example of the disparate treatment of black in conditions of employment" in Fairfax's county government, the brief said. The 91-page document was filed in Alexandria federal district court as the Justice Department began trial of a race and sex discrimination suit against the county that Fairfax County Board Chairman John F. Herrity tried -- and failed -- to get the Reagan administration to drop.

The suit, initially filed by the Carter administration, had been largely rejected by Judge Albert V. Bryan Jr. in April 1979. But an appeals court overturned the decision last July, and sent it back to Bryan for retrial, saying it found strong evidence that the county had discriminated against women and blacks.

County officials have bitterly rejected the accusations and Herrity, a Republican and the county's top elected official, had hoped the new GOP administration, despite public statements to the contrary, would end the case.

Yesterday, Assistant County Attorney Jack L. Gould returned to Bryan's courtroom with 14 cartons of documents which he said would refute the federal allegations.

Gould argued that black and female applicants were not hired in some cases because they were simply not qualified. He also charged that federal attorneys were "using data that was not refined" to make their case, and that entry-level exams for various county jobs did not discriminate against minorities as the federal government alleged.

"They the Justice Department must prove discrimination is a way of doing business in Fairfax County. It is not," Gould said.

"We will also show affirmative action data that is both striking and very favorable to the [county] in that in the period of 1979 and the first six months of 1980. In all EEO [Equal Employment Opportunity] categories, there was no statistically significant disparity in hiring," Gould said.

Gould argued that the Justice Department's statistics alleging gross disparities in the number of female and minority applicants and the number receiving county jobs failed to account for the possibility that some of the same individuals may have applied several times for one or more jobs.

Daniel Lanigan of the county's research office testified that more than 1,500 applicants in the period under scrutiny "applied twice, 592 applied three times . . . going all the way to one person who applied 50 times."

Len Ehrich, the county's public works director, testified that he has an African-born foster son and, at one time, was manager of the water and sewer system for Liberia.

"The best qualified applicant [for one job] was white, but I directed the agency to hire a black for the position who was qualified, but not as qualified. I have directed persons to hire minorities and give them assistance training for those who might not be as well qualified to make them better employes," Ehrich said.

Testimony in the case resumes today at 10 a.m. with several highranking county officials expected to testify, including Police Chief Richard King and Sheriff Wayne Huggins.