To demonstrate mechanical transmission of African trypanosomes by tabanids, a series of 3 experiments was carried out in Lahirasso, Burkina Faso. Ten heifers free of trypanosome infection, were kept together in a corral covered by a mosquito net. In the two first experiments, 2 heifers were experimentally infected with a local stock of T. vivax. Tabanids freshly-captured with 2 Nzi traps were released into the fly proof corral, during 20 days: on average 32 Atylotus agrestis / heifer / day in the first experiment, and 54 Atylotus fuscipes in the second one. Mechanical transmission of T. vivax was demonstrated in both cases with high incidence rates: respectively 63% and 75%. In a third experiment 2 heifers were experimentally infected with a local stock of T. congolense. On average 29 freshly-captured Atylotus agrestis per heifer / day were released in the same conditions. The incidence rate of T. congolense infection was 25%. Impact of tabanids as mechanical vectors of trypanosomes should be re-considered in light of these results.