Little information is available on the sensitivity of Latin American isolates livestock of trypanosomes to trypanocides. Resistance to diminazene aceturate and isometamidium chloride was evaluated in sheep and calves for 3 isolates of T.vivax from French Guyana, 1 from Venezuela and 1 isolate of T. evansi from Venezuela. Whichever treatment was applied, parasites were no longer detectable the day after the treatment. All the isolates were resistant to diminazene at the dose of 7mg/Kg except one isolate from French Guyana; relapses were recorded 12 to 150 days after the treatments. Sheep were shown to be potentially healthy carriers of T. vivax and T. evansi. Resistance was not recorded to the treatment with 0,5 mg/Kg isometamidium chloride in T. Vivax infected sheep; duration of the protection was evaluated in some of the animals by re-inoculation of the same isolates after every 15 days; the animals protected for more than 4.5 monthes. In T. evansi infections, although the infections could not be ascertained, either by direct examination of by mice inoculations, the antibody level appredated by indirect-ELISA T. evansi had not decrease 4 monthes after the treatment; it is suggested that this isolate of T.evansi is resistant to isometamidium chloride. The use of diminazene aceturate and isometamidium in enzootic and epizootic, monospecific or multispecific situations is discussed, and is opposed to the strategy generally applied in Africa. The role of reservoir of ruminant and dogs for T. evansi and eventually of horses for T. vivax is recalled.