none Female Bracon hebetor (Say) experienced limited reductions in fecundity and fertility when fed 0.05 and 0.1% 5-fluorouracil. Both sterility effects were associated with altered RNA metabolism in a combination of target cells not previously identified in earlier studies of 5-fluorouracil on arthropods or tests of the response of B. hebetor to related base analogs. Lower egg production resulted from generalized cell debility affecting nutrient uptake and utilization in treated females. Timing and nature of embryonic death in larvae, concurrent with a temporary lowering in hatchability, suggested the action of this compound was on nurse-cell RNA synthesis. In contrast, mature sperm cells were unaffected when they received similar treatments of 5-fluorouracil.