Deterministic models were employed to investigate the biology of Trypanosoma evansi infection in the Indonesian buffalo. Models were fitted to two age-structured data sets of infection. The Susceptible–Infected–Susceptible (SIS) model was the best supported description of this infection, although the results of the analysis depended on the serological test used: the Tr7 Ag-ELISA was judged the most reliable indicator of infection. Estimated forces of infection increase with age from 1·2 to 2·0 acquisitions per buffalo per year. The buffaloes would clear infection in an estimated mean time period of 16·8 months (95% CIs: 12·5–25·9 months) since acquisition, either by drug treatment by owners or self-cure. A general discussion on the role of immunity in protozoan infections includes consideration that the fitted SIS model would be consistent with strain-specific immunity. The model may become a useful tool for the evaluation of control programmes.