The microturbellarian fauna of North America is poorly known. In our survey of interstitial waters, stream surface sediments, and springs in southeastern New York, we found 33 species of flatworms, of which five probably are new to science and eight more are new to North America. The genera Chordarium, Ascophora, Limnoruanis, and an undetermined kalyptorhynch genus are reported from North America for the first time. Members of the order Catenulida dominate the microturbellarian community in all three habitats that we studied. The density of microturbellarians in interstitial samples varies seasonally, with lowest densities occurring in winter. In stream surface sediments, microturbellarians are most abundant in sand. Although microturbellarians are not a dominant part of the fauna in interstitial waters or stream sediments, they are encountered regularly in such habitats, and should be considered as an integral part of the fauna of these and other fresh waters.