Individuals of Centrostephanus coronatus, a diadematid sea urchin, restrict their grazing to relatively confined areas on subtidal rock surfaces. To determine the effects of Centrostephanus grazing on the composition and diversity of the attached biota within these areas, I removed sea urchins from an experimental site and photographically monitored subsequent events in marked quadrats at monthly intervals for 27 mo. Grazed and ungrazed control sites were similarly monitored. These observations were supplemented by dietary data obtained from grazed control site urchins at the end of the experiment, permitting assessment of food preferences through the use of electivity indices. Taxonomic composition in the exclusion site changed dramatically following urchin removal nearly converging to that of the ungrazed control site. Algae, sponges, tunicates, and erect bryozoa replaced encrusting coralline algae and encrusting bryozoa as the dominants. The urchin's diet includes all of these taxa, and the most preferred ones increase most upon urchin removal. Taxonomic diversity increased in the exclusion site as a result of replacement of a small number of taxa which can withstand urching grazing by a larger number of competitively superior ones which cannot. Thus the effect of Centrostephanus on the community is to decrease diversity where it actually grazes, but to increase diversity in the community as a whole by creating local environmental patches favorable to otherwise rare taxa.