Dissolved organic carbon (DOC) and NO3 - are important forms of C and N in stream water. Hypotheses concerning relationships between DOC and NO3 - concentrations have been proposed, but there are no reports demonstrating a relationship between them in stream water. We observed 35 natural streams in the Lake Biwa watershed, central Japan, and found an inverse relationship between DOC and NO3 - concentrations. This relationship was also found in observations of their seasonal variations in the Lake Biwa watershed. Moreover, this relationship was also found to apply to watersheds in other regions in Japan. These results suggest that forest biogeochemical processes which control DOC and NO 3 - concentrations in Japanese streams are closely related. Excess N availability together with a C (energy) deficit in a soil environment may explain this relationship. DOC and NO3 - concentrations in streams will thus be a useful index indicating C and N availability in catchments. © The Ecological Society of Japan.