In Tamaulipas, Mexico, and Florida, U.S.A., small-scale field tests were conducted with several insecticides applied to horses infested with Dermacentor nitens Neumann, the tropical horse tick. In Mexico, all stages of this one-host species were found in the ears of horses. The ears were treated by hand with insecticide in water, mineral oil, smears, dusts, and other formulations, and the bodies sprayed with the same insecticides in water. At 1 week after treatment, ticks were scraped from the horses' ears, and mortality counts made to determine the effectiveness of the treatments. Limited data indicated all treatments killed many ticks, but some differences in effectiveness were observed. Highly effective treatments were 0.5% carbaryl, 0.1% Compound 4072 (2-chloro-1-(2,4-dichlorophenyl) vinyl diethyl phosphate), 5% coumaphos dust, 1% dioxathion in mineral oil, EQ-335smear, and 5% ronnel smear; less effective Were(0.25% coumaphos, 0.75% lindane in ear tick remedy, and 0.15% dioxathion; least effective treatments were 0.05% diazinon, 0.03% lindane, 0.5% malathion, 0.5% ronnel, and 0.5% toxaphene. In Florida, the ears of horses were infested and in most cases all stages were found in the nostrils and manes; with some horses, ticks were found between the ears, and on the belly and tail. The ears and nostrils were treated by hand and the bodies sprayed. Horses were examined 1 and 2 weeks post treatment. More effective treatments for ears and nostrils were 0.5% carbaryl, 0.05% carbophenothion, 0.1% Compound 4072 5% coumaphos dust, 0.15% dioxathion, EQ-335 smear, and 0.75% ronnel; less effective treatments were 0.1% fenthion, 0.25% coumaphos, 0.03% and 1% lindane, and 0.5% malathion. In one small test, trichlorfon in feed for 10 days at 20 mg/kg/day and fenthion in feed for 5 days at 5 mg/kg/day were systemically effective against D. niteus in the cars of horses that consumed treated feed.