Small invertebrates in many phyla live in close association with Pelvetia fastigiata, a brown alga common in rocky intertidal areas of Southern California. Over linear distances of tens of meters, densities of P. fastigiata differ greatly because some plants are spatially isolated and others live contiguously within aggregations. Seasonal fluctuations in the faunas of P. fastigiata specimens collected between 1974 and 1977 in the Bird Rock region of La Jolla, California, USA, differed with the size, distribution and tide level of the plants. Such fluctuations coincided with variations in several environmental factors and their differences agree, in part, with the predictions of three hypotheses developed for the biotas of habitat patches and islands. The faunas of small and moderate-sized isolated P. fastigiata at mid-tide levels are disturbed by abiotic factors during the fall and winter and thus, the numbers of species on such plants tend to vary more than those on (1) larger plants at the same sites, or on (2) comparable plants within aggregations or (3) comparable isolated plants at upper mid-intertidal levels. The numbers of invertebrate species and individuals on plants at all sites examined, however, vary with changes in the net reproduction, immigration and/or loss rates of animals within several groups of species that inhabit P. fastigiata. The copepod Scutellidium lamellipes and the amphipod Ampithoe tea live and reproduce on P. fastigiata throughout the year, but are most abundant and, particularly on isolated plants, most widely distributed in the summer and/or fall. Other animals are most frequent or abundant in spring, early summer or even winter collections of P. fastigiata. Because many of the latter taxa typically do not inhabit aggregated and/or upper mid-intertidal plants, relationships between plant size and the total numbers of species and individuals on P. fastigiata specimens consistently differ between sites. Thus, although disturbance and temperatures may affect the faunas of P. fastigiata at all sites, between-site differences in faunal composition which have previously been related to habitat-patch size are maintained in all seasons.