The Mermithid nematode Romanomermis culicivorax Ross and Smith was released at a dosage rate of ca. 3.6 X 103 preparasites/m2 of water surface in an attempt to control mosquitoes breeding in a grassy field. Results from the release of ca. 9x 106 preparasites indicated that 96.5 + 2.8% of the 1st-4th instar Psorophora columbiae Dyar and Knab, Ps. ciliata F., Culex nigripalpus Theob. and Aedes taeniorhynchus (Wiedemann) larvae from 8 potholes and ditches in the field were parasitized by R. culicivorax, and marked reductions in host populations were observed in samples 24 hr post-treatment. No larvae were found in 2 additional sampling sites 24 hr post-treatment that contained high populations of 1st and 2nd instar Ps. columbiae and/or Cx. nigripalpus prior to preparasite release.