The proportion of males to females ofNeomesomermis flumenalis(Welch), andIsomermis wisconsinensisWelch, increases with increase in the number of nematodes per host. Collection of larvae ofProsimulium mixtum/fuscumSyme and Davies andSimulium venustumSay with 1, 2, 3, or 4N.flumenalisper host had 11.9%, 43.9%, 81.7%, and 93.8% male mermithids respectively. A similar increase in the proportion of males with an increase in parasite density was observed inI wisconsinensis. Most hosts were parasitized by a single mermithid.