Locomotion in terrestrial isopods is strongly influenced by body size and by abiotic factors. We determined the speeds of isopods of differing masses within a linear racetrack at temperatures ranging from 15 to 35 degrees C. We also predicted maximum speeds based on the Froude number concept as originally applied to vertebrates. In addition we used a circular thermal gradient to examine the temperature preferences of isopods, and we measured the effects of desiccation on locomotion. Measured speeds of the isopods progressively increased with temperature with an overall Q(10) of 1.64 and scaling exponents ranging from 0.38 to 0.63. The predicted maximum speeds were remarkably close to the measured speeds at the highest test temperature although the scaling exponents were closer to 0.15. The isopods did not exhibit a strong thermal preference within the gradient; however, they did generally avoid temperatures above 25 degrees C. Moderate desiccation had no apparent effect on locomotor performance, but there was a progressive decrease in speed once animals had lost more than 10% of their initial body mass. Though largely restricted to moist habitats, P. laevis can easily withstand short exposures to desiccating conditions, and they are capable of effective locomotion over a wide range of temperatures. Since they are nonconglobating, active escape appears to be their primary defense when threatened under exposed conditions. Although their maximum speeds may be limited both by temperature and by their inability to change gait, these speeds are clearly adequate for survival.