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We examined the effects of predation by Nigorobuna Carassius auratus grandoculis larvae and fry, a crucian carp endemic to Lake Biwa, Japan, on the community structure of aquatic organisms in rice fields. Six experimental plots with three different rice-straw treatments in the presence/absence of stocked larvae were prepared. In each plot, the number of aquatic organisms ranging in size from 30 lm to 5 mm in the water, as well as those from 63 lm to 5 mm in size in the surface sediments, was surveyed 6, 13, 20, 26, 34, and 41 days after the onset of irrigation. Three-day-old fish larvae were released on day 10. Undigested organisms in the gut contents of the larvae or fry were identified on days 20, 26, 34, and 41, respectively. Ten-day-old larvae mainly preyed on Cladocera, but the fry thereafter shifted to Diptera as their main prey. While Cladocera and Podocopida decreased in fishstocked plots, Euglenales and Halteriida became more abundant there. Top-down or bottom-up effects of fish seemed to control these changes in community structure.