Diurnal and seasonal variation in spatial distribution of fifteen species of fish were studied in Nicola Lake during summer. Maximum depth at which fish were taken was 110 ft. where only Kokanee were found. Peamouth Chub, Largescale Sucker and Prickly Sculpin were found in depths up to 80 ft. Chiselmouth were restricted to the shallow southwest basin all throughout summer. Carp fingerlings were also restricted to shallow weedy areas. All other species were available in all major regions of the lake. Young-of-the-year of Peamouth Chub, Largescale Sucker, Squawfish, Red-side Shiner and Prickly Sculpin stayed very close to shore during daylight. Fry of Kokanee, Rainbow Trout, Mountain Whitefish and Chinook Salmon appeared in areas close to stream inlets and outlets during day. Smaller size groups of fish available on the shore in daylight moved offshore at night, when larger fry and adults appeared in the shore. Young of Mountain Whitefish were dispersed in different regions of the shore at night. Variation was observed between daylight and dark distribution of different species. Adult Peamouth Chub and Redside Shiners stayed on the bottom during daylight but they invaded surface and shallow shore areas at night. Squawfish also tended to leave the bottom at night. Factors determining migration and summer distribution of fishes were investigated. Thermal stratification was unstable and had no apparent effect on vertical migration of most species. Only Squawfish avoided the hypolimnion. Dissolved oxygen was plentiful up to a depth of 96 ft. and its effect, therefore, could not be assessed. Light apparently played an important role in determining movements of fish in daylight and dark. Diurnal variation in distribution is attributed to effect of light. / Science, Faculty of / Zoology, Department of / Graduate