Laboratory studies were conducted to determine if there were biological or behavioral differences among populations of Trissolcus 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 of Nezara uiridula (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 from N. uiridula eggs than those from Italy or Spain. When male T. basalis were continuously present with females, fecundity, number of male progeny, and percentage of parasitism were significantly higher than when females were isolated. Over the lifetime of T. basalis females, 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 pop- ulation took significantly more time to complete immature development than the French or Spanish population. N. vtridula egg mortality was significantly higher when female T. basalis were 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 marking N. utridula eggs than those from Spain or France. The results of these studies suggest that the three populations compose a single T. basalis biotype.