The patterns of diel activity of four large decapod species in the shallow subtidal of the Isles of Shoals, Gulf of Maine, U.S.A. were investigated. During the summer of 1999 the diel abundance and size distribution of active decapod individuals were surveyed at three depth ranges at a sheltered site on Appledore Island. Densities of active American lobsters, Homarus americanus H. Milne Edwards, 1837, were, as expected, highest at night. The crabs Cancer borealis Stimpson, 1859 and Carcinus maenas (L., 1758), however, were almost exclusively active during the day. Cancer irroratus Say, 1817 were equally active during the day and the night, but the mean size of individuals was significantly larger during the day. Surveys at additional sites in 2003 confirmed that these same patterns of diel activity were present throughout the Isles of Shoals. An extensive review of the literature suggests that such diurnal activity is not only unusual for the three crab species of this study, but for the whole genus Cancer as well.