During one year, growth and mortality of the salt marsh gastropod,Littorina irrorata, were examined at three levels of population density within three salt marshes in the Morehead City region of North Carolina. Density was manipulated by enclosures. The standing crop of the dominant vegetationSpartina alterniflora differed among the three marshes. Overall weight gain ofLittorina differed among marshes, density treatments, and three size classes of snails. Weight gain in the smallest individuals was significantly different among the three marsh areas, but not the density treatments. For intermediate-sized individuals, growth rate differences occurred only in response to the density treatments. In adults, weight gain was different in the three marshes, but the density treatments had little effect. Within-cage mortality appeared independent of the density treatments, but significantly different among the three marshes. Computed equations characterized average annual growth in shell width ofLittorina irrorata in the three marshes.