Field tests were conducted with Romanomermis culicivorax Ross and Smith, against 4 species of mosquito larvae in 3 natural and 2 artificial sites. Infective nematodes were disseminated at 706 to 25,000 per m2 surface area. All species of mosquito larvae were infected and the percentage infection was dependent on the mosquito subfamily, application rate, and test site. In mixed mosquito populations Anophelines were more susceptible to parasitism than Culicines. Infections in Culicine mosquitoes did not exceed 62% at the highest exposure rates. Control of mosquito larvae was reduced in sites with dense vegetation or algal mats.