The effect of Simulium griseicolle on the blood of growing and laying birds was studied for periods of 10 and eight weeks respectively. The red cell counts and the haemoglobin concentrations were reduced but the mean corpuscular volume was increased. The packed cell volume and the mean corpuscular haemoglobin concentration were only slightly changed. The macrocytic normochromic anaemia thus observed in the infested birds might have been due to nutritional factors rather than to blood loss caused by the feeding insects. Morphological changes were observed in the erythrocytes of the infested birds and these suggest presence of in vivo haemolysis, probably caused by some haemolysing agents in the saliva of the insects.