Studies in various species have shown that changes in reproductive activity result in inverse changes in life span. It is interesting to know how this "cost of reproduction" is incurred. It is possible that reproductive activity renders the organism more vulnerable to stress and that accumulated damage has a role in the observed decrease in life span. Previously, other investigators had shown that mated female fruitflies have significantly shorter life spans than virgin females. We compare mated and virgin young fruitflies for susceptibility to lethal heat stress. Preliminary results suggest that mated fruitflies are significantly more susceptible to heat stress than virgin ones.