Immersion was used to test adults of six species of entomopathogenic nematodes for their effects on virulence and reproduction of engorged females of southern cattle fever tick, Rhipicephalus microplus (Canestrini) (Deutch strain). The nematodes were in the genera Steinernema (Rhabditida: Steinernematidae) -- Steinernema carpocapsae (Weiser) (All strain), Steinernema feltiae Filipjev (SN strain), and Steinernema riobrave Cabanillas, Poinar, and Raulston (355 strain), and Heterorhabditis (Rhabditida: Heterorhabditidae) -- Heterorhabditis bacteriophora Poinar (VS strain), Heterorhabditis floridensis Nguyen, Gozel, Koppenhöfer, and Adams (K22 strain), and Heterorhabditis indica Poinar, Karunakar, and David (HOM1 strain). Five engorged female ticks were immersed in a suspension of 5,000 infective juvenile nematodes per milliliter for various times (0.5 to 30 minutes), and the number of ticks killed over time, egg mass weight, reproductive index, and percentage of inhibition of oviposition were calculated. The percentage of ticks killed was affected significantly only by the nematode species; immersion time and species-by-immersion time interaction were not significant. The Heterorhabditis species killed more than did Steinernema nematodes, with H. floridensis having the greatest efficacy. The nematode species and immersion time significantly affected the reproductive index, whereas the effect of nematode species was not affected by immersion time. Heterorhabditis spp. had more pronounced effects on the reproductive index, with significantly lower values than by Steinernema spp. The percentage of inhibition of oviposition was significantly affected only by nematode species, but not immersion time, and species-byimmersion time interaction was not significant. Results demonstrated that infective juveniles of H. floridensis have potential for biological control in conditions such as dipping vats where cattle infested with R. microplus are immersed in a solution of entomopathogenic nematodes.