Individuals of Amphissa columbiana from the intertidal zone of San Juan Island, Washington, U.S.A., typically have either shells with very thin, delicate apertural lips, or shells with thick, robust lips. In laboratory observations, thin-lipped snails grew rapidly but were not sexually mature, while thick-lipped snails grew very slowly or not at all and were sexually mature. These observations are consistent with the hypothesis that A. columbiana displays determinate growth, as has been inferred for many columbellids on the basis of intraspecific variation in shell form. Sizes of mature snails were very variable, with the largest individuals weighing 4.5 times more than the smallest (wet weight, excluding shell). I tested the hypothesis that maturation and associated shell thickening are phenotypically plastic responses to the presence of predators. Exposure to effluent from the predatory crab Cancer productus in the laboratory had no effect on shell form or relative shell weight (an index of shell thickness), suggesting that this is not the case.