Reduction in locomotor performance during pregnancy is a potential cost of reproduction for female lizards and snakes. Most previous studies have suggested that reduced locomotor performance is a direct result of carrying a physical burden (the clutch). However, recent studies suggest that physiological changes associated with pregnancy can also cause a reduction in locomotor performance, particularly in viviparous species with prolonged gestation. I measured locomotor performance of the Northern Death Adder (Acanthophis praelongus), a viviparous, semiaquatic ambush foraging snake from tropical Australia. Swimming speeds of males and nonreproductive females were very similar, but late-term gravid females swam 30% more slowly than nonreproductive animals. Females with enlarged oviductal follicles carried physical burdens similar in size to those carried by gravid females but did not have impaired locomotor performance. Swimming speed of gravid females was unrelated to two measures of relative clutch mass. These results support the hypothesis that physiological changes during late pregnancy contribute to the decrement in locomotor performance during pregnancy. Because locomotor performance was uncorrelated with the level of reproductive investment, survival costs during pregnancy may be less important than previously thought, particularly if physiological costs during pregnancy render females vulnerable to starvation or predation following parturition. Additional field studies are needed to clarify the nature of survival costs in natural populations and their consequences for the evolution of squamate life histories.