Two-way factorial experiments were set up to compare the fecundity and longevity ofTrichogramma minutumRiley and the percentage emergence and sex ratio of their offspring when reared from the eggs ofEphestia kuehniellaZeller,Sitotroga cerealella(Olivier),Choristoneura fumiferana(Clemens),Manduca sexta(L.) orLambdina fiscellaria(Guenée) (rearing host species) and attacking the eggs ofE. kuehniella, S. cerealellaorC. fumiferana(host species attacked). Data were combined across all host species attacked when analyzing the effects of rearing host species and vice versa. There was no significant difference in any of the measured parameters for femaleT. minutumreared from eitherE. kuehniellaorS. cerealella, but greater fecundity and longevity were recorded for females attacking eggs ofE. kuehniellarelative to those attacking eggs ofS. cerealella. Females reared fromE. kuehniellahad lower fecundity than those reared fromL. fiscellaria, M. sextaorC. fumiferana. Parasitoids had significantly lower fecundity and longevity when attacking eggs ofC. fumiferanathan when attacking eggs of eitherE. kuehniellaorS. cerealella, and the percentage emergence of the offspring was significantly higher from the latter two host species. Parasitoids reared from eggs ofC. fumiferanaproduced more offspring on eggs ofE. kuehniellathan on eggs ofC. fumiferana. Results from these studies highlight the need to separate the effects of the host species from which the parasitoid is reared from the effects of the host species which the parasitoid attacks. They also illustrate the need to examine reproductive performance against the target species before a comprehensive cost-benefit analysis of a mass-rearing system is made.