The effect of grazing by a small herbivorous gastropod Homalopoma amussitatum on the density and species composition of benthie diatoms was examined in the laboratory. H. amussitatum is a dominant species on exposed crustose coralline algae (CCA) along the Sanriku coast in northeastern Japan, and it has been Suggested that it competes for food with postlarval abalone. The minimum densities of the gastropod required to inhibit the growth of diatoms were estimated to be < 10 g/m(2) at 20 degrees C, between 10-20 g/m(2) at 15 degrees C, and > 20 g/m(2) at 7 degrees C. In the absence of grazing pressure, Nitzchia sp. and Psamodictyon panduriformis. which have a growth form classified as Type A (gliding prostrate type), initially formed flat communities. Amphora sp., Cocconeis sublittoralis (Type B: adhesive prostrate type) and Tabularia investiens (Type C: nonmotile upright type) Subsequently increased in number. The two species of Type A were significantly affected by grazing pressure and did not increase even at a gastropod density of 10 g/m(2) at 15 degrees C and 20 degrees C. Types B and C diatoms were more tolerant of grazing by the gastropod than Type A species. However. the volumetric rate of ingestion of Cocconeis scutellum (Type B) by the gastropod was almost the same as for Novicula britannica (Type A) at the three experimental temperatures, when they were supplied to the gastropod independently. The digestion efficiencies of C. scutellum and N. britannica were 94.7% and 2.2%, respectively. The dietary value of C. scutellum for the gastropod H. amussitatum is thus considerably higher than that of N. britannica. In the natural CCA habitat, competitive grazing by H. amussitatum on the diatom Cocconeis spp., which is known as a major food for postlarval abalone, could significantly affect the survival and growth rates of postlarval abalone.