Mermithid nematodes are common parasites of black flies and play a significant role in the natural regulation of these medically important insects. Infection levels tend to he moderate and perennial, with epizootics rare and highly localized. Mermithid parasitism almost invariably results in the death of the black fly, and thus considerable attention has focused on the potential of these nematodes as biocontrol agents. Early instar black fly larvae appear most susceptible to infection, and integumental penetration hy mermithid preparasites is the only known mode of entry. Postparasitic nematodes typically emerge before host pupation. However, carryover of parasitism into adult simuliids is an important mechanism for local dispersal and recolonization of upstream areas. Following emergence, the mermithids molt to the adult stage. Copulation ensues, the females then laying eggs which eventually give rise to the next generation of infective preparasites. The number of described species is conservatively estimated at 35-40, with most species within the genera Mesomermis, Gastromermis, and Isomermis. The taxonomy of this group of mermithids is a challenging and little explored area. Host-specificity statements, therefore, must be made cautiously because of these systematic problems and others within the Simuliidae. In most instances, temporal and spatial factors limit the host range of these mermithids among simuliid species. Differential susceptibilities anmng larvae concurrently present within the same microhabitat probably reflect varying degrees of host attractiveness and behavioral-physiological resistance. Effects of parasitism on the host may include prevention of metamorphosis, sterility, intersexual development, and behavior modification. Evaluation of the technical feasibility of mermithid control of black flies has been stymied by the limitations of current inoculum-production technology. Continued advances in in vivo and in vitro culture methods are required to accelerate the research process.