The southern flounder Paralichthys lethostigma, host to the nematode Philometroides paralichthydis that is embedded in place of the inclinator muscles of the dorsal and anal fin elements, is hypothesized to impair two aspects of locomotor performance (swimming and burying capacity). Peak swimming acceleration and both measures of burying performance did not differ between infected and uninfected fish, whereas swimming velocity of infected fish was significantly lower than that of uninfected fish. Smaller infected fish swam at significantly slower speeds than smaller uninfected fish, whereas there was no difference among larger fish. Neither the location nor the number of worms affected either swimming or burying performance. The decrease in swimming velocity observed in smaller infected fish may be sufficient in rendering them more vulnerable to predation and environmental stressors.