Laboratory studies were conducted to determine if there were biological or behavioral differences among populations ofTrissolcus basalis(Wollaston) (Scelionidae: Hymenoptera) collected from France, Italy, and Spain. Observations of isolated and mated females indicated little difference in fecundity, adult longevity, male and female progeny produced, length of oviposition period, and percentage of parasitism ofNezara vttidula(L.) (Heteroptera: Pentatomidae) eggs. Females from all locations showed greater fecundity and rates of parasitism during the first week after emergence. The French population showed a significantly higher emergence rate fromN. viridulaeggs than those from Italy or Spain. When maleT. basaliswere continuously present with females, fecundity, number of male progeny, and percentage of parasitism were significantly higher than when females were isolated. Over the lifetime ofT. basalisfemales, the sex ratio of adult progeny favored female; however, there was a progressive shift from predominantly female to male progeny as the female aged. Adult males lived longer than females. No significant differences in adult male or female longevity were detected across geographic populations. However, the Italian population took significantly more time to complete immature development than the French or Spanish population.N. viridulaegg mortality was significantly higher when femaleT. basaliswere continuously present than when absent, indicating that parasites may cause some mortality in addition to that resulting from parasitism. There were no significant differences in mating, egg selection, or oviposition times among the geographic populations. Females from Italy spent significantly less time in markingN. viridulaeggs than those from Spain or France. The results of these studies suggest that the three populations compose a singleT. basalisbiotype.