Erythrocyte nuclear volumes were measured in brook trout (Salvelinus fontinalis) from Phillips Hatchery in Maine to document naturally occurring polyploid sterility. In eight brook trout in which gonads were lacking or undeveloped, the nuclei were large, suggesting polyploidy. The average size of the erythrocyte nuclei in other sterile fish fell within the normal range but all of the eight fish appeared to have some erythrocytes that were polyploid. All polyploids appeared to be mosaics, containing diploid, triploid, tetraploid, or pentaploid cells. The cause of the polyploidy was not determined but may have been caused by the inadvertent exposure of the eggs to low temperatures after fertilization.