Microhabitat selection and diurnal and nocturnal movement patterns in two tropical littoral gastropods (Nerita versicolor and Tectarius antonii) were compared in relation to shore position allowing for an assessment of the influence of physical stressors (heat and desiccation) on their behaviors and vertical distributions. Monitoring of individually marked specimens indicated that movement, and presumably feeding activities of tropical littoral gastropods, is initiated during periods of decreased desiccation and thermal stress. Near continuous movement was observed in snails inhabiting moist eulittoral (midshore) habitats. In contrast, snails inhabiting the upper eulittoral fringe (high-shore) exhibited movement only during periods of rain or when shores were moistened by increased wave action. Twenty-four hour surveys indicated a preference for nocturnal activity in mid- and high-shore habitats. The propensity for increased activity during conditions of reduced desiccation stress was also supported by an experiment in which wetting of the substrate (simulating inundation) encouraged activity in mid- and high-shore species. Mid- and high-shore species exhibited a preference for sheltered microhabitat and avoided exposed surfaces.