The response of summer and winter annuals to early summer irrigation and/or natural precipitation was determined for a rocky slope and a sandy bajada near East Ord Mountain in the Mojave Desert, California. Irrigation stimulated the germination of summer annuals on the rocky slope but not on the bajada, where a thundershower later in the summer caused a profusion of annuals. The following season, winter annuals showed markedly reduced densities on both irrigation plots and on the area receiving summer rain. These surprising results suggest that summer precipitation may have a much greater influence on vegetation response than had been previously supposed. Annuals comprised over 80% of the plant species on the two sites. Native winter annuals appear as components of highly integrated stable communities. The spatial heterogeneity of annual communities is greater than for perennial communities.