The object of this field study was to examine Nilsson's hypothesis (Nilsson, 1965, 1967) of interactive segregation as it might apply to feeding and spatial distribution of Cutthroat trout (Salmo clarki clarki) and Dolly Varden (Salvelinus malma) in allopatric and sympatric populations of the two species in small lakes. Salmo clarki clarki living alone changed their depth distribution throughout the summer season but no diel movement was recorded. The changes in food habits closely corresponded with seasonal change in distribution, and a wide variety of food organisms was eaten. Salvelinus malma living alone underwent considerable diel changes in spatial distribution, with the majority caught at the surface during the evening. Their diet consisted primarily of zooplankton and surface-caught insects. Sympatric Salmo clarki clarki and Salvelinus malma were spatially segregated throughout the summer. Cutthroat trout occurred mostly near the surface and in littoral areas and fed in these areas. In contrast, Dolly Varden were mostly benthic in distribution and fed primarily on bottom organisms. By comparing food habits and distribution in allopatric and sympatric situations it was inferred that food competition contributes to the segregation of the species in sympatry. Seasonal differences in food and spatial segregation occur suggesting interactive segregation is a temporary phenomenon. In general, the data support Nilsson's hypothesis of interactive segregation. / Science, Faculty of / Zoology, Department of / Graduate