An investigation into the trophic dependencies of fish and chironomids in the River Thames, U .K ., is described . The food of 15 species of larval chironomid was studied and the importance of chironomids as food for five species of fish was assessed . Very fine detritus, probably tripton, was the sole food item of planktonic first instar larvae . Fine detritus was the most important food item in the diets of most larval chironomid species and was even a major food item of the predacious Tanypodinae . Diatoms were the next most important food item ; filamentous green algae, coarse detritus and animal remains, although present in the gut, were not important numerically . It was concluded that three species were filter feeders, whilst the other species were nonselective detritivores . Chironomids were important in the diets of all fish species studied ; peak numbers were ingested in June . During the winter, fish ingested chironomids from the Acorus and flint zones . In the spring and summer, when large popula-, tions of chironomids were available in the Nuphar zone, the fish utilized these . This study underlines the importance of detritus in the river system, from the tripton to the fish via the chironomids .