Glyptapanteles flavicoxis (Marsh) a gregarious larval parasitoid of Lymantria obfuscata (Walker) readily attacks the gypsy moth, Lymantria dispar (L.). This species might have potential for inundative releases against gypsy moth populations, because it can be reared in large numbers with relatively few hosts. Unfortunately, sex ratios in laboratory reared G. flavicoxis are usually male-biased, hindering efforts to mass release it for biological control by making the production of females costly. Because parental age at time of mating affects the sex ratio in some Braconidae, we crossed males and virgin females 0-, 1-, 4-, 9-, and 16-days-old with at least 10 trials for each of the 25 combinations. Numbers and sex ratios of progeny produced by females each day were recorded and subjected to statistical analysis. Both progeny and sex ratios (% females) among progeny produced by ovipositing females of G. flavicoxis decreased markedly over time, so we focused our analyses on sex ratios in progeny produced on the first day hosts were provided to females. Females in all age classes mated to newly emerged males (day 0) were more likely to produce all male progeny (30%) than those mated to older males (10-15%). When crosses with only male progeny were excluded from the analysis, females mated to males one day old had higher sex ratios than those mated to males in other age classes. In addition, females mated the day that they emerged tended to have the highest sex ratios.