Water resource partitioning among three co-occurring species of the California annual grassland was investigated. Plantago erecta, Clarkia rubicunda and Hemizonia luzulifolia differ in lifespan. The lifespan of Plantago is coincident with the October-May rainy season, but the other two species reproduce during summer when no precipitation occurs, and thus depend on stored water.Field studies indicated differential access to stored water commensurate with the phenology of each species. Studies of artificial stands under controlled conditions showed no difference in the species' ability to exploit stored water in the soil. However there was a striking difference in root behavior between Plantago and Hemizonia when plants were grown in a soil layer above a non-nutritive, waterstoring substrate.We concluded that Hemizonia, the longest lived species, survives on water stored in decomposed rock below the soil layer. Clarkia is restricted to cooler slope faces where a slightly longer growing season appears just suficient to complete reproduction.Productivity is enhanced by addition of later blooming species to the community, but there is no indication that the mixture is the most productive system.