Spawn taken annually from rainbow trout (Salmo gairdneri) entering the inlet at Lake DeSmet constitutes Wyoming's primary source of trout eggs. The number of eggs taken has fluctuated between 500,000 and 4 million. Effects of planting rainbow trout of spring- and fall-spawning races and locations of plants were studied from marked fish. Rainbow trout of the spring-spawning race entered the spawning run in an 11-to-1 ratio over the fall-spawning strain. Fish planted in the inlet ditch returned in a 3-to-1 ratio over those planted in the far end of the lake. Anglers harvested marked trout randomly from all portions of the lake. Trout apparently migrated to the spawning trap from all portions of the lake. Ten percent of the adult trout shore-trapped and tagged in the spring migrated to the inlet ditch and spawning trap. Seventeen percent of females trapped in 1957 returned in 1958 to make up 13 percent of the female run. Most of the run is spawning for the first time. Creel census from January to June during 1958, 1959, and 1960 indicated harvest of trout was not large enough to affect the spawning run seriously. Between two and four trout per surface acre were harvested. Planted trout must be at least 4 inches long (40 per pound) to escape predation by yellow perch. An annual plant of 500,000 spring spawning trout appears sufficient to maintain the rainbow trout population. At least 40,000 of these fish should be planted in the inlet ditch to insure an annual take of 2 million eggs.