Partial creel-census records of Fish Lake, Utah, for the seasons of 1941–1943, were analyzed for evidence of a relationship between moonlight and fishing success. For both trolling and fly-fishing, the difference in the catch at times of new and full moons was not significant. The fly-fishing catch at times of first quarter moons (1.69 fish per man-trip) was significantly less than the mean catch at times of adjacent new and full moons (3.87 fish per man-trip); mean difference, 2.19 ± 0.675; value of t, 3.244. It is probable that light is the essential factor in producing the observed effect on fly-fishing at times of first quarter moon. This type of fishing is done almost exclusively after dark. A high ridge on the east shore of the lake protects the fishing area from the light of the full moon during effective fishing time, but the area is exposed to the light of the first quarter moon. Suggested explanations for the mode of operation of moonlight in reducing the catch are discussed.