Metaseiulus occidentalis (Nesbitt) is a phytoseiid mite which is commercially available as a biological control agent of spider mites. Genetic manipulation of this phytoseiid species has yielded transgenic strains, but none have been released into the environment. Previous data suggested that M. occidentalis could not survive the wet, humid summers in Florida. A non-transgenic strain of M. occidentalis was released into field plots in Gainesville on soybean plants infested with the two-spotted spider mite, Tetranychus urticae Koch. Populations were monitored from April-October 1994, and weather data were gathered at the release site. Permethrin-treated barrier rows were monitored to determine if the mites dispersed outside the plots, and aerial dispersal was monitored with sticky traps. Predator and spider-mite populations repeatedly crashed during the summer months, and population growth was negatively correlated with rainfall. CLIMEX, a population growth model which uses climatic factors to determine whether a given poikilothermic species can colonize and persist in new geographic areas, also indicated that M. occidentalis cannot persist through the wet season in Florida, although it may be able to establish and persist through the fall, winter and spring months.