The microstructure of the inner shell layer of Geukensia demissa (Dillwyn) varies with both the season of calcium carbonate deposition (or dissolution) and the latitude of sampled populations. Living specimens (n = 510) were sampled at monthly and, occasionally, biweekly intervals over a 3-year period from a natural intertidal population in Gulf of Maine estuarine waters. Scanning electron microscopic examination of the inner shell layer growth surface and fractured shell sections of the sampled specimens suggests that the observed structural changes reflect seasonal cycles of aragonite deposition and dissolution. Discrete nacreous tablets generally are deposited during the warmer months of the year from May through September. During the remaining months of the year, varying degrees of shell dissolution are apparent from ultrastructural examination of the inner shell layer growth surface and fractured shell sections. Additional specimens (n = 285) were sampled during various seasons from six geographically separated populations from Prince Edward Island, Canada to Cape Kennedy, Florida, USA. Examination of acetate peel replicas and polished thin sections of shells from northern populations has revealed the presence of three distinct types of aragonitic microstructure (granular, simple prismatic, and nacre), with an increased percentage of granular structure relative to that present in shells from more southern populations. Granular microstructure is absent in shells from North Carolina and Florida populations; in the inner shell layer of specimens from these southern localities, extensive regions of nacre alternate with horizons of simple prismatic aragonite. It is suggested that area percentages of the various microstructures (particularly granular), when viewed in consistently-oriented antero-posterior shell sections, may be highly correlated with mean annual seawater temperatures. Ecological and paleoecological applications of the observed microstructural changes in the inner shell layer of this species include temporal and spatial temperature gradient reconstructions, as well as analytical studies of demographic (and individual organism) growth rates and age structure.