Freshwater mussels are declining rapidly in many parts of their range throughout North America, primarily as a result of anthropogenic alterations of their habitat, including damming of rivers. To assess the effects of lowhead dams on freshwater mussel assemblages in the Neosho River, Kansas, we sampled mussels by groping along transects and searching haphazardly along a 100-m stretch at eight sites of four site types (i.e., upstream reference, upstream treatment, downstream treatment, and downstream reference) centered around two lowhead dams. We collected from four to 11 species of mussels at each site, and a total of 13 species. Analysis of variance indicated a significant difference in mean species richness and evenness, but not abundance, among site types, consistent with the hypothesis that lowhead dams affect freshwater mussel assemblage composition in the Neosho River.