Twenty-two breeding sites of Anopheles albimanus in the coastal area of El Salvador were treated with the Mermithid nematode Romanomermis culicivorax at dosages ranging from 2400 to 4800 preparasites/sq m of surface water. A mean parasitism of 96% was obtained for all of the treatments. No statistical differences were apparent between dosages or instars and levels of parasitism. Also dosages or instars and levels of parasitism. Also 2 early evening treatments of the Anopheline breeding area (1.2 ha) of Lake Apastepeque in November, 1977, with the nematode produced an average 96% parasitism. It was concluded that the increased time between the early evening treatments and wave action the next day allowed greater contact between the parasite and host and was responsible for the much higher level of parasitism than previously observed following midday treatments.