Odon. larvae collected in the Mignone River, central Italy are reported in correlation with the chemico-physical features of the sites sampled. All spp. found are shown to tolerate moderate natural mineralization of water in the absence of man-originated pollution. Results are compared with data for the nearby Topino River, which is more polluted, and with some other streams. Calopteryx splendens appears to be of little utility as a biological indicator, but other odon. spp. may be useful.