Blood samples for hematological and parasitological studies were obtained from 30 Atlantic cod (Gadus morhua) at intervals after infection with Trypanosoma murmanensis via leeches. Hematological procedures included estimation of hematocrit, hemoglobin, erythrocyte sedimentation rate (ESR), and differential white cell count. Tissues were also examined for histological changes. Results from the cod could be divided into two groups. In the first group of 10 cod (16–24 cm), coincident with the rise in parasitemia, hematocrit and hemoglobin levels decreased and the ESR increased following infection. Values did not return to normal levels by 92 days postinfection despite decreasing and somewhat irregular fluctuating parasitemias after 29 days. There was no apparent change in the differential white cell count. Increased hemopoiesis occurred in the kidney and various developmental stages of erythrocytic precursors were noted in peripheral blood. The fish appeared somewhat lethargic. In the second group of 20 cod (26–49 cm), hematological changes observed were slight and there was no evidence of decreased mobility. These results suggest that the blood changes observed in the group of small cod are sufficient to affect them similarly under natural conditions so that they could become more prone to predators, pollutants, or diseases. Key words: blood changes, Atlantic cod, trypanosomes