Infectivity of the mermithid nematode Heleidomermis magnapapula, a parasite of Culicoides variipennis variipennis and C. v. sonorensis in dairy wastewater pond habitats, was tested against larvae of aquatic Diptera in the laboratory. Observations were made on preparasite penetration. If the host species could be reared after parasitization, further observations determined nematode development and emergence. Genera in the families Syrphidae (Eristalis), Psychodidae (Psychoda), and Chironomidae (Tanypus), common in wastewater pond habitats, were not attacked. Larvae of Bezzia and Dasyhelea (Ceratopogonidae) also were not attacked. Larvae of Chironomus (Chironomidae) were penetrated, but rapidly encapsulated the nematodes. All Culicoides spp. exposed were readily penetrated by H. magnapapula preparasites. The nematodes successfully emerged from C. v. occidentalis. Some nematode maturation was seen in C. lahontan, and limited adult nematode emergence was seen in C. boydi and C. cacticola. No nematode development was observed in 5 other Culicoides spp. penetrated.