Romanomermis culicivorax Ross and Smith was tested against Culex quinquefasciatus Say and Culiseta incidens (Thomson) in both laboratory and field at 5, 10, and 20:1 inoculation ratios (preparasitic nematodes: mosquito larvae). In the laboratory, the preparasitic larvae infected 100% of the larvae of both species of mosquitoes at all inoculation ratios with the exception of Cx. quinquefasciatus at the lowest inoculation ratio (72% parasitism). In field applications in cemetery vases, only a 30% infection of Cx. quinquefasciatus was obtained at the highest inoculation ratio (20:1). Few Cs. incidens larvae (11%) were infected in the field experiment. Recycling of the nematodes did not occur in the cemetery vases. The low level of inoculation and lack of recycling in the cemetery vases were probably due to the low oxygen tension and high conductivity of the water in the vases.