A hematological study was made on 119 mountain whitefish (Prosopium williamsoni) from the Yellowstone and Madison River drainages in Montana, 1961–62. Hemoglobin determinations had a range of 7.0–13.7 g/100 ml (mean 10.6 g) while hematocrits had a range of 28.5–62.2% (mean 43.5%). Erythrocyte counts ranged from 1.01 to 2.34 million/mm3(mean 1.57 million) and leucocyte numbers ranged from 360 to 25,500/mm3(mean 5400). Thrombocyte counts ranged from 870 to 47,200/mm3of blood (mean 17,600). Leucocyte differential counts had a mean of 33.9% lymphocytes and 66.1% neutrophilic granulocytes with the predominant granulocyte being the lobed mature form. Granulocyte means were higher in leucocyte counts over 8100/mm3. Few immature cells were seen in blood smears. Some polychromatic erythrocytes were found and myelocytes were the most immature granulocytes seen. Differences were observed in the various values between small and large fish, males and females, and for fish taken in summer and winter. The kidney was the active hemopoietic tissue, forming granulocytes, lymphocytes, and thrombocytes. The spleen produced most of the erythrocytes and occasionally other cell types. The liver was hemopoietic in some anemic fish.