A strain of the entomopathogenic nematodeHeterorhabditis bacteriophora,genetically enhanced for thermotolerance by introduction of a heat-shock protein gene from the free-living nematodeCaenorhabditis elegans,was released in turfgrass field microplots in the spring, summer, and fall of 1996. As predicted, transgenic and wildtype strains did not differ in their ability to persist. We document the regulatory procedures at the federal, state, university, and local levels needed before field release, none of which posed any significant difficulties. Our risk assessment study supports the regulatory view that the transgenic nematode strain is an unlikely environmental threat. Subsequent regulatory reviews in the United States appear likely to continue to be decided on a case-by-case basis according to organism phenotype rather than the techniques used to generate them. This is the first report of a nonmicrobial, genetically engineered insect natural enemy being released into the environment.