Female Porcellio laevis (Isopoda: Oniscidea) brood eggs and young in ventral marsupial pouches, a reproductive strategy with pronounced physical and presumed fitness costs. This study examines the effect of physical stress on reproductive behavior. To induce excessive physical stress, gravid female P. laevis were subjected to five minutes of continuous locomotion daily throughout the brooding period. Stressed females exhibited significantly shorter brooding periods than controls. Although stressed and control groups did not differ in average body size, there was a strong trend of reduced fecundity in stressed females. These results indicate that female isopods modify critical elements of reproductive investment under conditions of physical stress.