Trout were introduced into a flow-through enclosure constructed in the headwaters of a small fishless stream to examine the effects of trout predation on the density, standing crop and production of lotic insect populations. Following the trout introduction, the density and standing crop of 3 of the 9 insect species examined decreased in the experimental stream study section (enclosure) while the density and standing crop of 2 insect species increased in the experimental stream section compared with the control stream section. Data on the food habits of the trout indicated that trout predation had reduced the density and standing crop of these insect populations in the experimental stream section. Competitive release was suggested as the reason for the increase in density and standing crop of the insect populations in the experimental stream section. The role of trout predation in structuring stream insect communities is briefly discussed. Production estimates of lotic species in the experimental stream section were not significantly different from those in the control section. Trout consumed only 0.4 times the mean standing crop or 9-10% of the production of prey species. However, trout were thought to be intensively grazing the available food supply. / Science, Faculty of / Zoology, Department of / Graduate