An investigation to determine the availability and selection of mayfly, Caenis kungu Kimm., as a natural source for food rainbow trout, Oncorhynchus mykiss Walb., was carried out in Naro Moru River from November 1986 to October 1987. Diel periodicity of drift of C. kungu was established to be an integral part of the ecology of Naro Moru River. The diel periodicity portrayed the bigeminus pattern whilst the drift throughout the year showed a weak bimodal pattern. Rainbow trout showed some degree of selectivity in their utilization of C. kungu and this selectivity did not strictly correspond to its availability in the drift. The trout was found to be very opportunist and ate what was available at the time. However, while C. kungu was an important natural food source for trout, it might not have been the most important food source for fish in Naro Moru River. There was no significant correlation between the C. kungu taken in the diet of the trout and C. kungu drift (r(s) = 0.084, P > 0.05).
