Standing stocks and net photosynthetic performances were determined concurrently for the 13 most abundant intertidal macrophytes from San Clemente Island, California over a four-season period. A summer cover maximum and winter minimum occurred for Colpomenia sinuosa (Roth) Derbes & Solier, Egregia menziesii (Turner) Areschoug, Gigartina canaliculata Harvey, Halidrys dioica Gardner, and Sargassum agardhianum J. Agardh, whereas Eisenia arborea Areschoug and Corallina officinalis var. chilensis (Decaisne) Kützing had fall maxima and summer minima in cover. Ulva californica Wille and Gelidium pusillum (Stackhouse) Le Jolis had maximal cover during spring. Pterocladia capillacea (Gmelin) Bornet & Thuret was the only species with peak cover in winter while three macrophytes, Lithothrix aspergillum J.E. Gray, blue-green algae, and Phyllospadix torreyi Watson displayed little seasonality in standing stocks. Highest net production rates were measured during summer for nine of the 13 species and minimum rates were recorded in spring and to a lesser extent during winter. Correlations between seasonal fluctuations in standing stock and productivity were evident for only eight species. Total daily community production reached a peak in the fall (1.22 g C fixed m −2 of substratum day −1) and declined sharply through winter to a spring low (0.47 g C m −2 day −1), closely paralleling changes in ambient water temperature. Blue-green algae, C. officinalis var. chilensis, P. capillacea, and E. menziesii contributed 76% of the total community primary productivity for the year. Seasonal patterns of photosynthetic performances were highly variable with a tendency for most species to attain peak daily photosynthetic rates coincident with higher temperatures and longer daylength.