ABSTRACT: Carbon and nitrogen isotopic composition of infaunal invertebrates from profundal and littoral zones of Lake Biwa, central Japan, was studied. Samples of chironomid larvae, Stictochironomus sp., showed large variations in both δ¹³C values (-64.0 to -23.0”) and δ¹⁵N values (-1.3 to 8.8”) and were significantly depleted in both ¹³C and ¹⁵N in comparison to other animals and primary producers from this lake. Moreover, δ¹³C values lower than -40” are unusual for freshwater food webs based on photoautotrophic sources. There was a strong positive correlation between δ¹³C and δ¹⁵N values of Stictochironomus sp., which clearly indicates a mixing of nutrition from 2 isotopically distinct sources. We presume that, other than photoautotrophic producers, only methanotrophic bacteria, which utilize isotopically light methane, could provide a source that is so depleted in both ¹³C and ¹⁵N. We suspect that the trophic association of some Chironomidae species with methanotrophic bacteria may turn out to be a widespread phenomenon.