Male-biased sex ratios in populations of parasitic wasps used in biological control are undesirable, because a low proportion of females can prevent the establishment of introduced species or hinder commercial production of species used for augmentative control. Studies were conducted on potential factors contributing to male-biased sex ratios that have been encountered in laboratory rearings of the braconid endoparasitoid Glyptapanteles flavicoxis (Marsh) using the gypsy moth, Lymantria dispar (L.), as a host. Sex determination in this wasp is arrhenotokous (haplodiploid). In the first experiment, we stored adults at 13 or 16°C and allowed them to mate at 20 or 25°C and found that sex ratios (expressed as percentage females) did not differ among progeny of parents; many females produced all male progeny, suggesting that they had not been fertilized. In the second experiment, females were exposed to hosts soon (0–60 min) after mating or 23–25 h later. Sex ratios were higher (less male-biased) in progenies of females provided with the rest period than in those which were not. In a third experiment, females were allowed to mate from one to four times with a given male. Although differences between these groupings were not statistically significant, the data suggested that more than two matings might depress sex ratios of progeny. An alternative analysis with only two groupings (1–2 matings and 3–4 matings) suggested that more than two matings might increase male bias of progeny. Therefore, we suggest that matings of this species be monitored in sleeve cages so that paired females can be removed and separated after copulation to diminish the likelihood of excessive matings, then given a rest period before they are offered hosts for parasitization.