The erythrocytic size method for determination of ploidy in the crucian carp, Carassius auratus (Linnaeus) collected from Hokkaido was examined. The ploidy of 17 fish was determined by somatic chromosome number.The mean major diameter of 20 erythrocytes per individual ranged from 13.0 to 14.4μm in 5 diploids, from 15.4 to 17.7μm in 7 triploids and from 20.6 to 22.2 μm in 5 tetraploids. The shape of erythrocytes tended to be more long and slender with the increase in ploidy. The distribution of the mean major diameter of 20 erythrocytes per individual was examined in 31 fish collected from Lake Akan and in 32 fish from the Tokoro River.They were divided into three groups according to their erythrocytic size.Karyotype analysis of 8 fish from the Tokoro River and erythrocyte shape suggested that the animals belonging to the first group, with the smallest erythrocytes, were diploids, while those belonging to the second group were triploids and those belonging to the third group were tetraploids.The critical size between diploid and triploid was 15 μm in mean major diameter of erythrocytes and that between triploid and tetraploid was 19 μm. Ploidy was determined for a total of 1, 534 samples collected from 24 sites throughout Hokkaido by the erythrocytic size method. They were composed of diploids (38.3%), triploids (59.5%) and tetraploids (2.2%). Triploid fish were collected from 22 collecting sites and were the most dominant in 18 out of the 22 sites.Tetraploid fish were found in only 6 collecting sites located mainly in Eastern Hokkaido.