A federal appeals court has rejected Fairfax County's request that the court reconsider its recent ruling reopening an employment discrimination suit brought by the Justice Department against the county.

The decision means that the U.S. District Court in Alexandria must go forward with a new trial on the suit, in which the federal government charged that Fairfax over a period of years clustered blacks and women in low-paying, less desirable jobs.

Last year, District Judge Albert V. Bryan Jr. dismissed the suit after saying that the county had undertaken a generally successful program to hire more women and minorities in higher job classifications.

After the county's hiring practices came under attack, the Board of Supervisors in 1977 narrowly adopted an affirmative-action plan that set goals for placing women and minorities in higher positions. The county had been criticized for placing most women in clerical jobs and most blacks in service and maintenance positions.

But last month, a three-judge panel of the Fourth Circuit Court of Appeals ruled that there was strong evidence that the county had committed discrimination. Judge Harrison L. Winter found that Judge Bryan had failed to consider employment figures that included many women and blacks who lived in metropolitan Washington but outside the county. The appeals court also said Fairfax did not file employment records, but instead kept them in a shoebox.