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Four groups (A, B, C and D) of 10 rats in each group were used to investigate relapse of infection in single and mixed trypanosome infections. Groups A served as uninfected control, groups B and C were infected with Trypanosoma brucei (Wamba strain) and T. congolense (Federe strain), respectively, and group D was infected with both species (50% of each species in the infective inoculum). Each infected rat received 1×106 trypanosomes by intraperitoneal injection of the inoculum. On day 14 post-infection, when the parasitaemia had reached the peak, infected rats were treated with diminazene aceturate at a dose of 10.5 mg/kg as a single intraperitoneal injection. All rats were aparasitaemic after treatment until days 42 and 49 post-treatment, when relapse of T. brucei infection occurred in two rats in each of groups B and D, respectively. All effectively cured rats remained aparasitaemic until the end of experiment on day 91 post-treatment. In conclusion, only T. brucei relapsed in 20% of treated cases in both single and mixed infections, whereas no relapse occurred in treated T. congolense infection, thus indicating that the interaction of T. brucei and T. congolense in mixed infections did not affect the curability of T. congolense infection.