On the rocky shores of the Gulf of Maine, the American lobster, Homarus americanus, the Jonah crab, Cancer borealis, the Rock crab, Cancer irroratus, and the Green crab, Carcinus maenas, compose a guild of highly mobile predators. Although the species are potential competitors that consume the same prey and utilize the same shelters, lobsters also prey on crabs (i.e., lobsters are intraguild predators of crabs). During daytime low tides, crabs are also preyed upon by Larus spp. gulls. In this study, I investigated the importance of avian and intraguild predation in influencing the diel and spatial (depth) patterns of decapod activity in the algal-covered lower intertidal and subtidal zones of Appledore Island, Maine, USA. During the summer months of 1999, the diel abundance and size distribution of active individuals was measured at several depths for each decapod species. Densities of active lobsters were highest at night and did not vary with depth. Contrary to prevailing knowledge, C. borealis and C. maenas were active almost exclusively during the day. While diurnal C. borealis were significantly more abundant in the deepest zone (9-11 m), C. maenas was not found at this depth and was most abundant in the shallowest zone (0-1 m). Day and night C. irroratus densities were not significantly different and showed no significant variation with depth. Only C. irroratus populations exhibited a diel difference in the mean size of individual crabs. The mean size of individuals observed during the day was larger than those observed at night. During the day, C. borealis were larger at depths of 9-11 m than at depths of 1-3 m, while C. maenas crabs were larger at depths of 5-7 m than at depths of 0-1 m. The magnitude of avian predation was assessed by censuses of crab remains collected in the intertidal during periods of low tide. Results reinforce previous studies in suggesting that gulls are a major factor limiting the upper distribution of C. borealis. In the subtidal, the relative availability of potential shelters does not appear to account for the depth distributions observed among the species. The presence of a potentially sheltering Codium algal canopy at 1 to 7 m may be important for C. maenas and C. irroratus. A subtidal tethering experiment with C. borealis and C. irroratus revealed no diel or depth differences in crab survival. Overall, rates of predation were very low, but were significantly higher for small than for large crabs. Besides lobsters, no other predators were observed at the study site; predation on tethered crabs was therefore attributed to lobsters. In laboratory experiments exposing pairs of small and large C. borealis and C. irroratus to lobsters, small crabs were shown to be significantly more vulnerable to predation than large conspecifics; C. borealis and C. irroratus of similar size were equally vulnerable. Large crabs, however, were attacked 71 times more often than were small crabs. The results of this study reinforce concerns that traditional models of rocky shore community organization must be amended to include mobile predators. They indicate that competitive interactions between small crabs and lobsters may be unimportant relative to intraguild predation. For large crabs, perceived nocturnal intraguild predation pressures may be just as important as competitive interactions between guild members, especially in the recent absence of predatory fishes. These results underline the importance of understanding the ecological history of the community. They suggest that the overfishing of coastal fish populations has increased the relative importance of intraguild interactions and has thereby indirectly strengthened the link between marine and terrestrial ecosystems.