Carbaryl and its major hydrolysis product, 1-naphthol, were tested singly and together at concentrations ranging from 2 to 20 mug/ml towards photosynthesis, in vivo nitrate reductase activity and nitrogen fixation, and ultrastructure of a diazotrophic cyanobacterium, Nostoc linckia, isolated from soil. All three physiological processes tested were drastically affected and correlated well with the alterations in the ultrastructure of the organism. Combination of carbaryl and 1-naphthol, at different concentrations, interacted significantly, yielding additive, antagonistic, and synergistic responses.