Teneral tsetse flies, Glossina morsitans morsitans (Mall) aged less than 32 h were divided into two groups and infected initially with Trypanosoma congolense IL 1180. Flies of group A were maintained on healthy rats and those of group B fed exclusively on parasitaemic rats, to ensure regular re-infection. Flies in group A recorded mesoprocyclic, metacyclic and vectorial competence (VC) index values of 0.65 ± 0.09; 0.73 ± 0.10 and 0.47 ± 0.11, respectively. The corresponding values for group B flies were 0.76 ± 0.08; 0.79 ± 0.08 and 0.60 ± 0.10. In group A, the mesoprocyclic and the VC indexes of the females were significantly higher than those of the males. In group B, no such significant differences were registered between the sexes. For both sexes taken together, the mesoprocyclic and the VC indexes of the tsetse flies of group B were more important than those of flies of group A. Males of group B revealed a higher VC index than that of group A males; no significant differences in this index were observed in the females of both groups.
