Spatial distribution of Chironomid larvae in stony, sandy, and muddy substrates in the Santa Ana River was studied from March to June 1975. Riverine drift samples were also collected during the daytime to study the nature and magnitude of Chironomid drift in the river. Larvae belonging to 10 genera were taken in the benthic collections; 9 of these also occurred in the drift. In benthos, Tanytarsus spp. Chironomus spp. and Cricotopus spp. collectively formed 95% of the total midges. The sandy and muddy substrates supported 8-10 times more larvae than the stony substrate. Tanytarsus spp. and Cricotopus spp. were predominant in the sandy substrate, while Chironomus spp. was associated with the muddy bottom type. No significant difference was found in the density of Tanytarsus spp. prevailing in the stony and muddy bottoms. Cricotopus spp. was significantly more numerous in stony than in muddy substrate. Chironomus spp. was taken in smaller numbers from sand than mud and was absent from the stony habitat. In the drift collections, only Tanytarsus spp. and Cricotopus spp. were of quantitative importance, forming 79% of the total larvae collected during the study. The mean numbers of larvae taken in 2-hr drift catches were low, ranging from 18-40 only. Among midge pupae, Cricotopus spp. predominated in the drift.