The triploids of carp (Cyprinus carpio) have been induced at a maximum frequency of 91.7% by the cold-shock treatment of the eggs shortly after fertilization. The erythrocytes and their nuclei of the triploids had changed to long ellipsoids that had grown more in the major axis than the minor axis, and with a surface area of 1.44 and 1.40times, respectively, than that of the diploids. The number of erythrocytes of the triploids had decreased to 0.6 times than that of the diploids.