Heterotic behavior in the form of increased parasitization rates, total fecundity, total progeny, and host destruction was measured in crosses between cultures from two South American populations of Muscidifurax raptorellus Kogan and Legner. Determinations of proportional activity increases in hybrids varied with the calculation method, population-based values probably giving more reliable estimates of field performance. A Microsoft BASIC computer program to summarize parasitization activity and to calculate population statistics was developed for solitary and gregarious parasites. Heterotic behavior resulting when newly introduced parasite strains interbreed with resident populations might facilitate the field establishment of desirable selected traits, thereby improving performance in biological control.