Three series of critical tests were completed on a combined total of 46 horses to determine the efficacy of single doses of trichlorfon against bots, ascarids, pinworms and large strongyles. Different formulations of trichlorfon were administered by tubing intragastrically, mixing with the daily grain ration, injecting i.m. or pouring on the back at dose rates between 20-100 mg/kg. Administration by feeding tended to be more efficacious for removal of bots and less toxic to the horses than administration by stomach tube. In many of the tests, trichlorfon was given in the grain ration at the dose rate of 40 mg/kg of body weight, and the aggregate average removals of 2nd and 3rd instars of Gasterophilus intestinalis and G. nasalis in the 3 series of tests were between 97-100%. Removal of Parascaris equorum was equally efficacious with both the intubation and the grain feeding methods of dosing, and at the dose rate of 40 mg/kg, the aggregate averages were 99 and 100% in the 3 series. Removal of Oxyuris equi was variable.sbd.aggregate averages were between 11 (1 infected horse in the initial series) and 96 (5 infected horses in the 3rd series) to 100% (7 infected horses in the 2nd series). Large strongyles, Strongylus vulgaris and S. edentatus were almost completely refractory to the 40-mg/kg dose rate of trichlorfon. Dose rates of 40 mg/kg and less were generally well tolerated by the critical test horses. Higher dose rates (60 and 80 mg/kg) administered by stomach tube induced moderately severe to severe colic and diarrhea, whereas a dose of 80 mg/kg given in the feed resulted in only a transient softening of the feces. Likewise, 5 consecutive doses, 1 wk between doses, of a bolus formulation given at the rate of 80 mg/kg to 4 horses were well tolerated. Clinical trials involving a total of 2294 treatments of trichlorfon at dose rate of 35-40 mg/kg in pregnant and nonpregnant mares, stallions, suckling and weanling foals, yearlings and horses in training on 38 farms in central Kentucky [USA] did not cause notable adverse clinical effects.