Tidal marsh snails,Melampus bidentatus, from the Barn Island marsh in Stonington, Connecticut, showed a lunar (4-week) periodicity to egg laying during the 1992 reproductive season but a semilunar (2-week) egg-laying rhythm in 1993. These different egg-laying patterns were observed under laboratory conditions that were identical during the 2 years, as well as in the field. This fact suggests that environmental conditions existing prior to the reproductive season may be important in regulating the egg-laying periodicity. During the 1993 season, snails from four other southern New England marshes also exhibited a semilunar egg-laying rhythm; however, variations on this pattern were observed. Data accumulated during this study are consistent with the hypothesis that fecundity ofM. bidentatusis size specific.