The polymorphic, seaweed-dwelling intertidal prosobranch gastropo Littorina obtusata was studied at a sheltered shore in North Wales from March 1975 to October 1977. Shell shape changes with size, assists camouflage, and resists crab predation; there is no sexual dimorphism. Sex ratio rises with increasing size to a 20-30'/ excess of males in small adults, then declines below unity in the largest adults. Juvenile and adult shell colour morph ratios fluctuate with time and over short distance Morph proportions change substantially with age. There is no vertical migration. Population size frequency structure is usually trimodal. Adults constitute a stable peak at c. 114 mm and account for 50-60'/ of the populatii The rest consists of two actively-growin juvenile modes representing two year classes; recruitment is broadly continuous, with a winter peak. Pro. bability-paper analysis suggests that juveniles grow steadily through winte: then accelerate in spring to reach adult size in late summer or autui approximately 18-2L1. months from hatching. Growth rate is approximately 0.12 mm/week. Fifteen mark-and-recapture batch-tagging programmes using two or more 1-mm shell size cohorts provide size-specific and seasonal mean growth rates. Growth is fastest in early spring and slowest in late summer and autumn. Mean growth rate varies threefold with size, increasing from 6 mm to approximately 0.03 mm/day at 8.5 mm, then declining steadily through 14.5 mm. Differences between adult sexes are slight. Mean growth patter. approximates to the von Bertalanffy equations. Survivorship is independen of size (and, in adults, sex) and unlike growth does not vary predictably with season. Two individual-numbering capture-recapture programmes reveal no differences between male and female growth patterns, growth rates, or survivorship. Individual growth is not always asymptotic, and may proceed in widely-separated steps. Size does not necessarily indicate age. Similar-sized snails growing concurrently and together may differ sevenfold in von Bertalanffy growth parameter k (c. 1x 10-3 to 7x 10-3 days-1) and by more than an order of magnitude in growth rate (c. 2x 10-3 to 3x 10-2 1 day). Lifespan is three years or more.