The abundance of foraging bats (Pipistrellus pipistrellus and Myotis daubentonii) and flying insects over two rivers in north-east Scotland (Dee and Ythan), differing eightfold in nitrate levels, were compared by paired samples over three periods in summer 1995, during each of which 10 nights' sampling was undertaken using ultrasonic detectors and insect suction traps. Thirteen out of the 18 pairwise comparisons of bat passes, insect numbers, biomass and diversity were not significantly different. The oligotrophic Dee supported higher numbers of bats than the eutrophic Ythan in June, but this was attributed to an effect of adverse weather conditions affecting the Ythan. In July and August–September, neither bat abundances nor the abundance of Chironomidae and Trichoptera, the main food of the bats, differed significantly between the two rivers, although the Ythan showed higher total insect biomass than the Dee. The Ythan also showed a much higher abundance of small Diptera, mainly Psychodidae and Cecidomyiidae, associated with elevated nitrogen levels in the intensively farmed river valley. Thus a small eutrophic river can support as many bats and as high an insect density as a large oligotrophic one.