Specimens of Isomermis wisconsinensis, Gastromermis viridis, Mesomermis flumenalis, and Mesomermis camdenensis have been found in 14 out of 28 studied sites. Their hosts were the following blackfly species: Cnephia dacotensis, Cnephia mutata, Prosimulium sp., Simulium decorum, Simulium tuberosum, Simulium venustum/verecundum complex, and Simulium vittatum. Superparasitism was observed in several simuliid larvae and, occasionally, with 2 different mermithid species. The prevalence of different mermithid species varies according to seasons: Mesomermis genus is more abundant during winter, whereas Isomermis and Gastromermis are found in higher number during summer. The study of environmental parameters related to mermithid presence or absence shows that only stream depth makes a difference.