During 2007 and 2008, we surveyed freshwater mussels with timed searches at 35 sites in the Niangua River basin, an Osage River tributary in west-central Missouri. Our objective was to determine the distribution, species richness, and abundance of freshwater mussels in the basin. We observed a total of 714 live individuals from 20 species, including the Missouri endemic and species of conservation concern Lampsilis brittsi. The mean catch per unit effort (live mussels/person-hour) was 12 with values ranging from 0 to 144. Eurynia dilatata was the most abundant species (387 individuals observed, relative abundance = 54.2%), but all other species were present at much lower numbers. Eurynia dilatata and Venustaconcha ellipsiformis were the most commonly encountered species, both occurring at 24 sites. Our observation of 20 species is lower than historical richness in the basin (32 species), and nearly all species were formerly more widely distributed in the basin based on the occurrence of weathered and subfossil shells. Together with low catch per unit effort at most sites, these data suggest a sharp decline in mussel populations throughout the basin over the last few decades. This decline is cause for concern, but the causes are unknown.