We surveyed the mussel fauna of Twin Creek in southwest Ohio and examined its distribution relative to several selected environmental parameters. Unionid presence was determined with visual searches at 36 sites across 76 krn in four counties. Fifteen species from the subfamilies Anodontinae, Ambleminae, and Lampsilinae were recorded, with nine species found alive at 17 of the 36 sites. Mussel species richness and species composition agree in general with historical studies of Twin Creek and other streams in the Great Miami River watershed, although some species have declined or been extirpated. The subfamily Ambleminae was almost completely missing from the area. The presence of three species only as dead and relic shells also may indicate a loss of species richness over the last few decades. Species richness values from Twin Creek were lower than for streams in central and northeastern Ohio.