A localized storm on 9 May 1977 produced deluge-level quantities of rain (2.56 cm within 3 h) causing flooding of the rocky shoreline at Corona del Mar, California. Unlike the patterns observed throughout 1975 and 1976 and at a comparable site 40 km to the north during the same time period, the sea urchin Strongylocentrotus purpuratus experienced a significant decrease in cover (from about 2.0% to less than 0.1%, P < 0.05, ANOVA) in the lower intertidal zone (MLLW to +0.3 m) and disappeared entirely within the +0.3 to +0.6 m interval. Belt transects documented an average of 90.5% mortality, whereas a census of the total area between the two permanent transect lines revealed 93.6% of the S. purpuratus to be dead. A biotically similar area beyond the periphery of the region flooded (20 m north of the north transect line) experienced only 1.1% mortality of S. purpuratus. Ephemeral macrophytes characteristic of disturbed environments, Ulva californica/ Enteromorpha sp. (combined) and Ectocarpaceae, increased significantly in mean overall cover (14.6% and 8.9%, respectively) following the flood, as did newly recruited barnacles. However, the majority of persistent macrophytes, such as Hydrolithon decipiens, blue-green algal crusts, and Gelidium coulteri/pusillum, showed slight (but not significant) declines in mean cover (1.3%, 7.8%, and 5%, respectively). Therefore, stochastic events can have highly-localized species-specific catastrophic effects on intertidal populations which may set in motion subsequent changes to overall community structure that would be difficult to understand if a program of infrequent sampling was used.