ABSTRACT: We investigated spatial and temporal variation in the diet of a planktivore, the Cassin's auklet Ptychoramphus aleuticus, in the Gulf of the Farallones, California, USA, where its population has decreased ca 50% in size coincident with the generaldecline of zooplankton in the California Current. Stomach samples were collected and compared among birds foraging at sea during the non-upwelling, post-spring transition (to upwelling), and upwelling periods each year from 1985 to 1988. Upwelling-periodsamples also were compared with samples regurgitated to chicks on the Farallon Islands during that period; such samples had been collected during 1971 and 1977-1991. During the non-upwelling period, auklets fed on the small, energetically inferior,oceanic euphausiid Euphausia pacifica, but following the spring transition, the auklets switched to small, juvenile fish (mostly Sebastes spp.) until these had grown too large for capture. Thereafter, the auklets switched to the larger andenergetically more valuable euphausiid Thysanoessa spinifera, which had become available upon the onset of upwelling. This general pattern was repeated each year, although some between-year variability among ancillary prey was evident. Aukletforaging locality, however, varied only slightly throughout the year; auklets were most dispersed during El Niño. It was apparent that the availability of T.spinifera was linked to upwelling. The decrease in the prevalence ofT.spinifera in the diet fed to chicks from 1971 to 1991 is consistent with decreased intensity of upwelling in the region, as is the reduction in numbers of auklets in the region over the same period. Our results indicate the value ofstudies of easily accessible marine predators, which can temporally and spatially integrate patterns in the prevalence of prey species, thereby increasing our understanding of the important processes that structure food webs.