We investigated a casually recognized pattern of seasonal abundance exhibited by a population of the simultaneously hermaphroditic anomalodesmatid bivalve Lyonsia hyalina (Glassy Lysonia) at Bluff Hill Cove, Galilee, RI, and quantified the distribution of these intertidal to subtidal individuals from June 1993 through April 1994. Density in late spring averaged less than 2 individuals m^-2 and was followed by a summer explosion of up to 200 individuals m^-2 in localized areas, with an observed patchy distribution. A subsequent massive autumn mortality occurred, when the population returned to the previous low numbers. We propose several factors including predation, reduced temperature, and natural senescence as causes of the autumnal decline.