Laboratory assays indicated that infective-stage juveniles of Neoaplectana carpocapsae are highly pathogenic to Simulium spp. larvae. Instar susceptibility increased with larval size, with early instars being nonsusceptible. High rates of mortality (75 – 100\%) were achieved in assays against late instars. These results indicate that N. carpocapsae may have potential value as a blackfly biocontrol agent.