The reproduction, growth, mortality, and physical tolerances of the individual Dicathais aegrota are described and integrated with the results of a population study on intertidal reef platforms at Rottnest I. Although exposed to severe environmental stresses, the population on the reef platform remained relatively stable in size and age structure during the period of this investigation. A high mortality rate (greater than 70% of the population per annum) was counterbalanced by continuous recruitment of juveniles and the immigration of a small number of adults, both from the sublittoral area. The discovery that this species extends into the sublittoral area and relies upon that segment of the population to maintain those upon the reef platform, indicates a need for review of existing concepts of mortality and survival of intertidal communities.