EACH sex is part of the environment of the other sex. This may lead to perpetual coevolution between the sexes, when adaptation by one sex reduces fitness of the other. Indirect evidence comes from experiments with Drosophila melanogaster indicating that seminal fluid reduces the competitive ability of sperm from other males, thereby increasing male fitness(1,2). It also reduces a female’s propensity to remate and increases her egg-laying rate(3). In contrast to these benefits to males, seminal fluid has substantial toxic side effects in females, with increasing quantity leading to decreasing female survival(4,5). Here I show that when female D. melanogaster are experimentally prevented from coevolving with males, males rapidly adapt to the static female phenotype. This male adaptation leads to a reduction in female survivorship, which is mediated by an increased rate of remating and increased toxicity of seminal fluid.