Summary 1.In most animals, females live longer than males, and large individuals live longer than small individuals. We examine both gender and body size effects on adult lifespan in two species of seed beetles ( Callosobruchus maculatus and Stator limbatus ), and test whether body size differences between males and females can explain genderdifferences in mortality rates and lifespan. 2. In both species, the effect of age on mortality rate was best described by a logistic mortality curve. C. maculatus females have a higher initial mortality rate than males, while males have a much higher rate of increase of the mortality rate with increasing age. In contrast, S. limbatus males had a higher initial mortality rate (opposite to the pattern seen in C. maculatus ) but males and females did not differ in the rate of increase of mortality with increasing age. 3. Body size was positively correlated with lifespan in both species, but it explained very little of the variance in lifespan ( R 2 < 0·09 for both species) and did not explain the gender difference. 4. Lifespan was heritable for both genders of both populations ( h 2 between 0·19 and 0·55), and the shape of the mortality curves varied among families within both species.