The population ecology of Patella vulgata L. was studied as part of an oil pollution baseline study. The reproductive cycle was determined; recruitment, growth, and mortality were studied in detail in nine populations, and samples were collected at up to four tidal levels on a variety of shores. Maturation of the gonads began in July-August and spawning was in late September 1972 and in January-February 1974. Recruitment was influenced mainly by the wetness of the habitat. There was little evidence of annual fluctuations in recruitment success, except on a local scale. Growth rate and maximum length attained were highest on bare rock and lowest among closely packed barnacles. Growth was also favoured by wet conditions. Changes with time in growth rate and size attained were related to changes in biological habitat. There was very little seasonal variation in growth rate. Mortality was divided into juvenile and adult phases. Juvenile mortality was very great in wet sites but somewhat less in the presence of fucoids. On dry sites with barnacles, there were fewer juveniles and these lived longer and suffered less mortality. Most adult limpets died of old age, and longevity and growth rate appeared to be inversely related. During the period of study, there was a net loss of limpets from the nine populations being considered in detail.Changes in numbers · m-2, wet weight · m-2, and mean length by weight were related to tidal level and exposure to wave action. Much variation was encountered in these and in length-frequency distributions: this was related, in part, to changes in biological habitat, but temporal changes also seemed to be important. Population structure reflected population history.With increasing exposure to wave action, P. vulgata is almost totally replaced by P. aspera. On exposed shores, the two species grow at similar rates and P. aspera appears to become dominant because it suffers less mortality among juveniles. At low tidal levels, the growth rate of P. aspera does not change with increasing shelter, but P. vulgata grows faster and becomes the dominant limpet species. At higher tidal levels the growth rate of P. vulgata is less variable, and the distribution of P. aspera seems to be limited by its greater susceptibility to desiccation. The better survival of juveniles of P. aspera may be a critical factor in the development of high-density, small-size populations at the presumed centre of its range.Cyclical changes between limpets and fucoids appeared to be in progress on some of the shores investigated, and it is argued that certain shores are incapable of supporting a stable limpet population; on a very local scale there are continual fluctuations. The absence of the southern species P. depressa may be a critical factor in this.The ability of the survey to detect subtle long-term changes is briefly discussed. Temporal change and instability are the main unpredictable factors affecting the results, and the survey would be improved by continued regular surveillance.