A few studies have shown that male and female invertebrates differ in immunity and that these differences appear related to differences in sexual dimorphism and gender differences in life histories. Melanotic encapsulation of foreign objects in insects is one form of immunity. The damselfly Lestes forcipatus Rambur is moderately sexually dimorphic, and much is known about patterns of mass gain in congeners relating to differences in life history between males and females. In this study, females were more immunoresponsive than males under controlled temperatures, following emergence, and at a time when parasitic mites were challenging these hosts. However, males and females that overlapped in mass at emergence did not differ in their immune responses. Males in better condition at emergence were more immunoresponsive than lighter males, but this relation was not found in females. Sex differences in immune expression may have implications for how females versus males are able to deal with challenges from parasites, under varying environmental conditions.