The vectorial competence (VC) of teneral (less than 32 h) Glossina tachinoides Westwood and G. palpalis gambiensis Vanderplank, fed simultaneously on a guinea-pig infected with Trypanosoma brucei brucei EATRO 1125, was assessed. Statistical analysis of the experimental results revealed that female G. tachinoides had a significantly higher midgut infection rate than males. Such a sex-related difference was not observed in G. p. gambiensis. Male G. p. gambiensis had higher midgut infection rates than male G. tachinoides. The metacyclic index did not differ between both subspecies, although G. p. gambiensis showed relatively more metacyclic infections than G. tachinoides. A global VC of 0.0242 and 0.0483 was found for G. tachinoides and G. p. gambiensis, respectively. VC did not differ significantly either between sexes or between the two species. However, G. tachinoides more rapidly infected the feeding host than G. p. gambiensis. In all infected flies, the procyclic index value was superior to the metacylic index value, suggesting that the infection is established by an ascending origin. Both large and slender parts of the salivary glands were constantly infected. Longitudinally dividing trypomastigotes of unequal length have been observed in the alimentary canal of the flies.