SYNOPSIS. In many intertidal species, size-distributions vary with elevation of the beach. When this pattern is the result of behavior, it is appropriate to investigate its adaptive significance. In Olivella biplicata larger animals live higher on the shore and smaller ones lower. The inclusion of a bottom-swimming larval stage in the life history may enable new young to reach the lower shore, while settled snails segregate by size in response to light. These responses carry smaller animals to levels of the shore where physiological stresses lie within their range of tolerances. In moving higher on the shore, the larger snails, whose physiological tolerances are broader, exploit additional resources and reduce intraspecific competition. The size-class distribution also facilitates reproduction and augments a non-random mating pattern. Possible further values of the pattern of distribution are considered, e.g., a widened intertidal range may enable populations to survive violent fluctuations in the physical environment.