The dynamics of naturally occurring high (zone 1) and low (zone 2) populations of the limpet, Acmaea scabra, were investigated on Bodega Head in Sonoma County, California. Populations at different intertidal heights can be considered separate because migration between levels is precluded by the homing behavior of A. scabra. Each population was investigated by means of 400—cm2 quadrats containing marked individuals; the quadrats were censused approximately every 2 months and yielded information on density, size distribution, dispersion, recruitment, growth, and mortality. Samples were collected from each population approximately every 2 weeks to determine the seasonal pattern of reproduction and changes in size—specific weights. In zone 1 the probable pattern of food availability was distinctly seasonal because of the interaction of tides and climate. Growth and reproduction in this zone were also seasonal, probably increasing and decreasing with the food supply. Seasonal changes in size—specific weights were correlated with the reproductive cycle. Dispersion in zone 1 was always contagious. Recruitment was lower in zone 1, resulting in lower densities and a lower rate of mortality. Annual growth was greater in zone 1 because of the lower densities; consequently limpets in zone 1 were larger and biomass was greater than in zone 2. The population in zone 1 was susceptible to density—independent (catastrophic) mortality and corresponds to a population on the "edge" of the species' range. In zone 2 food was probably more constantly available because this zone was submerged more regularly and to a greater extent than zone 1. As a result there were more constant rates of growth and reproduction, and since reproduction was less variable, there were no seasonal changes in size—specific weights. Dispersion in zone 2 was random or, occasionally, uniform. Recruitment was higher in zone 2 resulting in higher densities and a higher mortality rate. Annual growth was lower because of the higher densities; consequently limpets in zone 2 were smaller and biomass was lower than in zone 1. The population in zone 2 was less susceptible to density—independent (catastrophic) mortality and corresponds to a population in the "center" of the species' range. Zone 2 is the "center" of the species' range, yet individual performance was poor because of the high densities; the probability of dying was high, growth rates were low, and the animals remained small. In both zones competition for food limited biomass, but lack of food did not cause mortality, i.e., regulate density.