Abstract: Chromosome segregation in fertilized eggs of pearl oyster , Pinctada martensii Dunker ,following the first polar body (PB1) inhibition with cytochalasin B (CB ) was observed. Cytological observation indicated that the inhibition of PB1with CB changed the normal chromosome segregation pattern. Polypolar segregation patterns were detected in the second meiosis ( M Ⅱ) . There were three typical patterns , namely dipolar segregation (28. 67 %) ,tripolar segregation (40. 56 %) and tetrapolar segregation (23. 78 %) in 3n ♀×2n .group . There were 6. 99 % of treated eggs that could not be classified. When chromosomes divided and segregated equally to each pole in dipolar segregation , tetraploids would be produced , and part of tetrapolar segregation also could produce tetraploid embryos. Aneuploids mainly resulted form triploid segregation . In the untreated group , chromosomes went through normal dipolar segregation ,and two polar bodies were released , but the chromosome number in each polar was highly variable , which gave rise to a variety of aneuploid embryos. The similar chromosome segregation patterns were observed in the group of diploids crossing diploids by blocking PB1 , including dipolar segregation (25.00 %) ,tripolar segregation (21.50 %) and tetrapolar segregation (34.30 %).The remaining 19. 19 % could not be classified. The possible mechanisms for the various types of chromosome segregation are discussed.