The redside shiner (Richardsonius balteatus) was introduced into Paul Lake, British Columbia about 1945 and by 1952 was abundant in the lake. Some effects of this introduction on Kamloops trout (Salmo gairdneri kamloops), previously the only other species of fish in the lake, are described. The summer diet of trout underwent marked changes and in 1952 shiners constituted a main item of diet for trout longer than 30 centimeters, fork length. Average stomach volumes of the smaller trout decreased subsequent to introduction of shiners. Growth rate declined and trout did not reach legal size (roughly 20 centimeters) until they were more than 2 years old. Previously, yearling trout had contributed substantially to the fishery. Change in growth rate has intensified the selection by the sport fishery of faster growing fish. A decline in the success of fishing and a drop in angling pressure have resulted in a decrease in catch to less than one-half of the previous annual total. These changes are discussed in relation to predation by shiners on trout fry, predation by trout on shiners and competition for food between shiners and trout.