In an attempt to identify the causes of the natural population fluctuations on British rocky coasts, an exploratory programme on the recruitment fluctuations of certain ‘key species’ has been operating for several years. The choice of species and the emphasis upon their recruitment have been discussed elsewhere (Lewis, 1976) and were justified as the most economical means of gaining the ability to (a) ‘explain’ major changes in the general character of rocky shore communities, and (b) make predictions about natural events against which to detect regional or temporal consequences of man-made changes in water quality. Mussels, barnacles and limpets are being studied but it is the last with which most interpretative progress appears to have been made, and this paper is concerned solely with Patella vulgata L. It is, however, essentially a preliminary report in a continuing study and it illustrates the practical and interpretative problems inherent in formulating and substantiating hypotheses in this type of work.