Entomopathogenic nematodes are a promising alternative for tick control, especially to protect wildlife. Two entomopathogenic nematode species Steinernema riobrave Cabanillas, Poinar, and Raulston and Heterorhabditis floridensis Nguyen, Gozel, Koppenhöfer, and Adams were tested for in-vivo efficacy against southern cattle fever tick, Rhipicephalus (=Boophilus) microplus Canestrini. Twelve heifer calves were experimentally infested three times at 1-week intervals with 125 mg of larvae of southern cattle fever ticks on days 0, 7, and 14. On Day 21, the animals were randomly divided into three treatment groups and sprayed with water only (check) or 5,000 infective juvenile S. riobrave or H. floridensis nematodes per milliliter. Number of ticks dropped, reproductive index, percentage of inhibition of oviposition, and hatching rate were calculated. S. riobrave and H. floridensis affected all life stages of the tick, and caused 14.5 and 25.4% reduction in production of adult engorged female ticks during the first 21 days post treatment. The nematodes significantly affected the reproductive parameters of treated ticks, but not egg hatchability. To our knowledge this is the first study to show a negative impact on ticks by entomopathogenic nematodes applied directly to the mammalian host of the tick. But, additional research is needed to enhance tick control using entomopathogenic nematodes. Novel formulations that enhance penetration or survival by nematodes should be investigated.