The present paper reports information obtained between 1982 and 1989 in the tropical lowlands of the Atlantic coast of Colombia. The studies confirmed that Trypanosoma vivax was widely but unevenly distributed within the study area and that incidence ranged from sporadic transmission to localised epidemics in which virtually all susceptible animals become infected within a period of 3 months. Quantitative epidemiological methods substantiated impressions of previous workers that T. vivax infections were associated with low-lying swampy areas and tabanid activity. Overt clinical trypanosomosis was rare in herds in which T. vivax was endemic. However, in these herds primary T. vivax infections were shown to cause subclinical alterations in calves. The infection consistently caused a temporary depression of packed cell volume (PCV) as well as a reduction in growth rate. There was no evidence of compensatory growth at a later stage.