Knowledge of predation intensity and how and why it varies among communities appears to be a key to understanding of community regulation. Along the rocky shores of New England, predation intensity in the mid intertidal zone appears to be low with exposure to severe wave shock, low desiccation stress, and a sparse cover of canopy algae, and high at areas protected from waves, with high desiccation potential and a dense cover of algae. As a result, predators at exposed headlands have no controlling influence on community structure, while at protected sites, they exert a strong and controlling effect on community structure.Experimental-observational studies of the effects of wave shock and desiccation on survival, foraging range and activity of the primary predator in this community (Thais lapillus) indicate that: (1) wave shock is a continuous and actual source of mortality at exposed sites but is relatively unimportant at protected sites; (2) mortality rates from desiccation at protected sites are potentially high and greater than at exposed sites; however, (3) actual desiccation stress is greatly reduced at protected sites by a dense algal canopy; (4) mortality from desiccation is greater in the higher mid intertidal than in the lower mid intertidal. Comparisons of activity patterns of Thais from April through November (these snails are usually active from May to early October) at an exposed and a protected site suggest snails at the former site restrict their active feeding to crevices while those at the latter site forage throughout the habitat. Field experiments support this hypothesis. Hence, differences in predator effectiveness at exposed and protected communities are probably due in part to the influence of wave shock. Exposed areas receive frequent severe wave shock in all seasons, even summer. Thus, the risk of being swept off the shore for snails foraging away from the shelter of a crevice at such areas is apparently great and exerts a strong selective force on foraging range. The importance of waves as a selective agent is further reinforced by the fact that crevices are nearly barren of prey, while just a few cm beyond the limits of the crevice, prey occur in great abundance.In contrast, at protected sites wave shock is never as severe as at exposed sites and is a relatively minor factor among several which might affect the foraging activity of a Thais. A major factor which varies among protected sites is the algal canopy. The influence of this factor is considered in a companion paper.