As invaders to North America, zebra mussels (Dreissena polymorpha) have contributed to the decline of benthic species, the mechanisms of which are poorly understood. In this study, we examined effects of D. polymorpha biofouling on locomotion of one pleurocerid snail (Elimia livescens) and two unionid bivalves (Ligumia nasuta and Anodonta grandis) using observational studies and removal experiments. In field and laboratory experiments, E. livescens without zebra mussels traveled 1.5–1.6 times farther than snails with zebra mussels. In field experiments, unionids without zebra mussels traveled farther and were more likely to move than unionids colonized by zebra mussels. Decreases in travel distance in the snail, but not unionids, were significantly correlated with zebra mussel loading relative to host animal mass. Limited mobility caused by zebra mussels may have consequences for gastropods and unionids by disrupting reproductive habits, predator evasion and avoidance of unbearable environmental conditions. Localized extinctions may occur due to inability to disperse or energetic costs associated with carrying zebra mussels. Mobility constraints resulting from zebra mussel biofouling may be an important mechanism of population declines in native mollusk species.