Feeding by Acroneuria (Calineuria) californica Banks was studied from May 1973 through April 1974 in a single riffle of Squirrel Creek, Nevada Co., CA. The stonefly nymphs contained an avg. 5.18 prey items each, with monthly means ranging from 1.66–19.19 prey/stonefiy. Gut content diversity was correlated significantly with benthic diversity. The most important food items were members of the Diptera, Trichoptera, and Ephemeroptera, generally accounting for more than 70% of the stonefly gut contents. Consumption by the A. californica population increased from May through October. Population consumption, decreased in November with decimation of the benthic community by washouts, remained low through April. Consumption of benthic insects by A. californica nymphs has a significant impact on the dynamics of its benthic prey. It is suggested that large populations of A. californica nymphs can reduce trout production in the field as well as in the laboratory.