Study has been made of substratum as a factor in the distribution of 5 species of pulmonate snails in Douglas Lake, Chebogyan County, Michigan. Quantitative field sampling, together with laboratory experiments, revealed that adult Physa integra Haldeman prefer hard substrata such as stones; adult Helisoma antrosa percarinata (Walker), in contrast, prefer a substratum of sand. When food (algae or detritus) was present on both stones and sand in the laboratory, the average distribution of P. integra was 58% on the stones and 8% on the sand (a highly significant difference), while in separate equivalent experiments an average of 24% of the H. antrosa were on the stones and 39% on the sand (also significant). Physa parkeri "Currier" DeCamp, Stagnicola emarginata angulata (Sowerby) and Helisoma campanulata smithi (Baker) all exhibited more complex and varied patterns of distribution as to substratum in the field. In the laboratory, P. parkeri showed a preference for a stony substratum very similar to that of P. integra when food was present. The average distribution of H. campanulata was 29% on stones and also 29% on sand when food was present on both. Relationships in the 5 species between substratum, on the one hand, and depth, wave action, food, oviposition sites, and respiratory needs, on the other, are discussed.