Lifetime oviposition was measured for femaleDelia antiqua(Meigen) caged individually with cups of moist sand or surrogate onions possessing combinations of host-plant color and chemical stimuli. Females laid more eggs on surrogates than on sand and laid more eggs on surrogates coated with synthetic onion volatiles (n-dipropyl disulfide, Pr2S2)than on untreated surrogates. Surrogate color had no effect on fecundity. Flies deprived of exposure to Pr2S2took longer to initiate oviposition and took longer to complete ovipositional cycles. The presence of untreated surrogates increased the percentage of eggs laid in ovipositional cups from 14 to 74%, whereas the presence of Pr2S2or host-plant color resulted in nearly all eggs being laid in cups. The number of eggs retained at death did not differ among treatments. The results suggest that differential oviposition may have resulted from reduced rates of egg maturation as a consequence of accumulation of unlaid eggs. Host specialization by the onion fly is probably due to ecological factors as well as an ovipositional preference for onion.