Although gastropods are important members of freshwater communities, the geographic range, ecological requirement, and conservation status of most species are poorly known. To advance this understanding, I used survey data from museums and peer-reviewed literature to summarize knowledge of the taxonomic composition and geographic distributions of freshwater gastropods in Iowa, U.S.A. Excluding records likely based on erroneous reports, 49 freshwater gastropod taxa (47 species and 2 genera with unknown numbers of species) inhabited Iowa during all or part of the period when records were collected (1821-1998). The Mississippi River and nearby tributaries of eastern Iowa and the prairie pothole and kettlehole regions of northern Iowa historically supported a large number of taxa. In contrast, few gastropods have been reported from the loess soils ecoregion of southwestern Iowa. Although recent improvement in water quality and increases in wetland habitat have likely benefited many gastropod taxa, it appears that as many as 18 species are now imperiled or extirpated from Iowa, and an additional 7 species were much less widespread at the end of the 20th century than formerly. These 25 species of conservation concern were identified on the basis of rarity or absence of recent records and on evidence of local extinctions that were associated with pollution and habitat loss. By comparing data summarized in this review with future data from field surveys, evidence or restricted or shrinking geographic ranges can be provided, and the true conservation status of Iowa gastropods will be determined. This information is of critical importance in establishing legal protection and action plans for the recovery of endangered species.