The DD-136 strain of Neoaplectana carpocapsae Weiser was tested in a small stream against black fly larvae at a dosage of 34.5 nematodes/ml over a 15 min exposure period. Mortality averaged 50% among late instar larvae treated in the stream and subsequently held in the laboratory under simulated stream conditions. Death was rapid: larval mortality commenced within 2-4 hr, and 64% of all deaths occurred within 24 hr of treatment. The intensity of infection in black fly cadavers was related to the day of larval death, with the heaviest infections observed in larvae dying within 1-2 days after treatment. Larvae dying toward the end of the 6 day observation period were lightly infected or uninfected. Treatment did not result in significant mortality among early and mid instars. Low stream temperature (9-12C) was probably the main factor limiting nematode effectiveness. No evidence of nematode establishment or recycling was detected.