The developmental environment can have lasting effects on posthatching phenotype in oviparous animals. Innate immune response is one important component of fitness in vertebrates because it provides a generalized defense against infection. In addition, because male vertebrates are at a higher risk of infection than females, males may benefit more from increased innate immunity than females. We determined the effects of incubation temperature on the innate immune response of hatchling map turtles (Graptemys) by incubating eggs at a range of male and female producing-temperatures and assessing plasma complement activity in the resulting hatchlings. We found a significant effect of incubation environment on circulating complement in hatchling Graptemys ouachitensis, with male-producing temperatures yielding the highest innate immune response. Most important, these results demonstrate that immune response is affected by developmental environment in a species with environmental sex determination, potentially resulting in sex differences in the ability to fend off pathogens.