Cultures containing the Mermithid nematode Reesimermis nielseni were introduced into habitats of floodwater mosquitoes in Louisiana as a pre-hatch treatment. Significant levels of parasitism were achieved when cultures were placed in damp habitats that possessed adequate amounts of vegetation or organic debris and were subjected to minimum flushing action when flooded. Fifty-two percent of Aedes atlanticus, 59% of A. tormentor, 38% of Psorophora columbiae, and 51% of P. howardii were parasitized in 39 larval collections from 13 habitats. Reesimermis nielseni was observed to penetrate but failed to develop in larvae of Psorophora ferox.