In the oocytes of the silkworm, Bombyx mori, only the sex chromosome bivalent (ZW) can be identified among the 28 bivalents, when we use the females of the sex-limited yellow cocoon (Sy) strain, in which the W carries a long fragment of the second chromosome. Autotetraploid silkwonns were induced from Sy females, in order to investigate whether preferential pairing between Z and Z as well as W and W occurs in the 4n oocytes. Two types of pairing patterns, (ZZ + WW) and (ZW +ZW), were observed in the oocytes, and the ratio of the pairing patterns, (ZZ+WW): (ZW + ZW), was 34:21 in Sy4n (Sy♂× Sy♀), while 16:2 in Syr4n (Sy♂× re9♀). Sex chromosome formulae of the eggs formed by the tetraploid can he confirmed when tetraploid females are crossed with sch males (sex-linked chocolate larva). The triploid off spring from Syr4n females mated with sch males, showed ZW gametes and ZZ gemetes produced in a 22:l ratio, which coincides with the ratio calculated from the pairing pattern ratio, 16:2. The results indicated preferential pairing of Z with Z and of W with W in tetraploid oocytes.