Trypanosome infections of Glossina pallidipes were investigated at a site in the Luangwa Valley, Zambia between June 1991 and September 1992. Almost 3700 flies were captured, dissected, screened for trypanosome infection, and aged using both wing fray and (for females) ovarian categories. DNA probes were used to identify midgut infections. Prevalences of mature infections were 6.2% Trypanosoma vivax-type and 3.1% T. congolense-type (including low prevalances of T. brucei, T. simiae and another Nannomonas species). The prevalence of infection increased with age. For both types of infection this relationship could be described by a simple 'catalytic' model which generates estimates of per capita rates of infection. Prevalences varied significantly with time independently of any changes in the age structure of the tsetse population. This may reflect temporal variation in the per capita rates of infection.