The productivity of 76 newly imported pregnant Friesian cattle and two bulls under heavy trypanosome challenge in the rain forest belt of Nigeria is reported. At the first visit in August 1989 and within 7 months of arrival of the heifers, the herd population had reduced by 26 (33.3%) as a result of deaths (six animals) and culls/salvages (20). The surviving 52 animals were generally in poor health with classical symptoms of trypanosomiasis. Thirty-one (40.8%) of the pregnancies were unsuccessful because of abortions (13 animals), premature births (seven), embryonic deaths (five) and death of heifers (six). Of the 45 successful calvings, 16 perinatal deaths occurred. All serum samples were negative for brucellosis. Only 41 (63.1%) of the 65 productive heifers lactated of which 24 (58.5%) yielded milk only for 6 months and less. The remaining 17 (41.5%) heifers were still at different stages of lactation ranging from 3 to 7 months within the period of analysis. Treatment with isometamidium (Samorin) at 0.5 mg/kg body weight cured the infection and prevented reinfections and/or relapses within 3 months of administration. A rise in the haematocrit and milk production after Samorin treatment was recorded. Careful analysis of the outbreak indicated that the reproductive wastages and poor lactational performance may have been induced by the severe trypanosomiasis diagnosed in the herd.