Year-round quantitative field sampling during 1970-72 revealed a clear seasonal migratory cycle in the Physa integra Haldeman in Douglas Lake, Michigan. In late April and early May, after the winter ice broke up, populations of this snail moved from water 2 m or more deep toward a cobble shore; there the adult snails spawned and later died. The new generation grew rapidly and, in September, began moving again into deeper water where the snails overwintered; they completed their migratory cycle the following spring. Field dispersal experiments conducted on adult snails in June revealed that some individuals of this species can move 7 m or more in 24 hr, that uphill movements predominate on an underwater slope, and that the direction of movement is also influenced locally by subsurface water currents, with the snails tending to move upstream. Laboratory experiments with adult snails confirm the upstream orientation and movement. Additional laboratory experiments on adults reveal a strong positive response to light, a negative response to gravity, and a tendency for the snails to move when on a sandy substratum and to remain more sedentary on a stony substratum where food is present. The adaptive significance of this behavior is discussed.