Female terrestrial isopods carry eggs and young throughout early development, a habit that places constraints on reproductive success. One such constraint is impaired locomotion during the brooding period. Brooding and non-brooding females were subjected to a negative-phototaxis experiment in which females moved away from a light source along a graduated surface. In both groups, velocity was positively and significantly correlated with distance traveled. Velocity and distance were also significantly associated with the physical dimensions of the exoskelton: larger females moved greater distances at faster speeds. Non-brooding females, however, moved significantly farther at significantly greater velocities than brooding females, suggesting that the additional mass of eggs and young restricts locomotion. The implications of body size and locomotion on reproductive success are discussed.