In the region of Barnstable Harbor, Massachusetts, 67 species of larger invertebrates were found in 3 faunal associations characteristic of different tide levels. The number of species per sample was highest at the intermediate tide level. Temperatures of sediments and densities of several species were followed for a year at two intertidal levels (60 cm and 90 cm above mean low water). Temperature variation was lower at the 60—cm level than at the 90—cm level at all times of the year, and at both levels was higher in spring than at any other time of the year. In three of the four species studied, spatial variation in density and rate of density change was significantly higher at the 60—cm location than at the 90—cm location. The mortality rate for adult G. gemma was higher at 60 cm (where density was lower) than at 90 cm, while the reverse was true for young—of—year. Release of young began sooner in the spring at 90 cm than at 60 cm. The growth rate of G. gemma at Barnstable Harbor was much lower than that reported at Union Beach, New Jersey, although at both places virtually all growth took place in summer. At Barnstable Harbor the maintenance requirements were 20—30 times higher in summer than in early spring. The Q10 within the temperature range 5—20°C was 4.1. G. gemma at 90 cm had less time to feed than at 60 cm, but had higher requirements for population maintenance because of an older age structure and a higher density. The growth rate of G. gemma was higher at 90 cm than at 60 cm.