We evaluated the effects of marine iguanas, sally lightfoot crabs, and fish on rocky-shore sessile organisms at two sites at Santa Cruz Island, Galápagos Islands, Ecuador, for 3-5 years during and after the 1997-1998 El Niño, using exclusion cages to separate the effects. Plots exposed to natural grazing were dominated either by encrusting algae or by red algal turf and articulated corallines. Algae fluctuated in response to El Niño in the following way. During an early phase, crustose Gymnogongrus and/or red algal turf were dominant. In the heart of El Niño, grazers had limited effects on algal cover but influenced algal sizes substantially. Most algae (particularly edible forms) were scarce or declined, although warm-water ephemeral species (notably Giffordia mitchelliae) flourished, increasing diversity and overgrowing crusts. Iguana mortalities were high, and crab densities low. When normal conditions returned, warm-water ephemerals declined, crab densities rose, and grazers had significant but site-specific effects on algae. At one site, any combination of grazers diminished most erect species, reducing diversity and restoring dominance of competitively inferior grazer-resistant crusts. At a second site, only the combined effect of all grazers had this effect. Laboratory experiments confirmed that crabs could control erect algae and promote crustose forms, and crustose Gymnogongrus developed into an erect form in the absence of crabs. Differences between sites and large-scale temporal changes associated with El Niño indicate that tropical shores are not all as constant in time and space as previously suggested. Mobile grazers did affect algal communities, but over the period of our observations far greater effects were attributable to intersite differences and temporal shifts in oceanographic conditions. El Niño events reduce nutrients, intensify wave action, and raise sea levels, affecting food availability for intertidal herbivores and their influence on benthic algae. Thus, the dramatic transformations of communities during El Niño presage the impacts of global climate change.