Haematological investigation was conducted on 20 cases of bovine trypanosomiasis caused by Trypanosoma congolense and T. vivax in imported Holstein-Friesian cattle using standard techniques. Comparison of the results with the values obtained for clinically normal animals whose blood samples were consistently negative for trypanosomes indicated that natural bovine trypanosomiasis was associated with a significant (P less than 0.001) fall in haemoglobin concentration, packed cell volume and numbers of erythrocytes. Slight reduction in the number of leucocytes, eosinophils, monocytes and basophils was recorded. No significant change (P greater than 0.001) was observed in the number of lymphocytes and neutrophils between naturally infected and control Holstein-Friesian animals.