Based on polyploid induction method in shrimp established in our laboratory , triploid Chinese shrimp, Fenneropenaeus chinensis was induced by heat shock to inhibit meiosis ? . Triploid shrimps and their diploid siblings were challenged w ith white spot syndrome virus (WSSV) through oral administration or with Vibrio anguillarum by injection. Shrimps without pathogens challenge were used as controls. Hemocyte count and composition in hemolymph of diploid and triploid shrimp were compared. This is the first time to report the cell count and composition of hemolymph in triploid shrimp. It showed that pathogens challenge resulted in obvious haematological changes. A signif icant decline for hemocyte count in both diploid and triploid shrimp af ter 4 days challenge was observed. Hemocyte count of both diploid and triploid shrimp was reduced to about 10% of that of control. The composition of hemocytes either in diploid or triploid shrimp changed after WSSV challenge. The amount of large granule cells ( LGC) decreased, while that of small granule cells ( SGC) increased. However, the amount of hyaline cells (HC) remained unchanged. Some abnormal structures such as vacant vacuoles were found in SGC and LGC of the infected shrimp. Hemocyte count of diploid and triploid shrimp decreased too after being infected with Vibrio anguillarum, and the range of decline was positively related with the injection dosage of Vibrio anguillarum. Triploid shrimp has larger hemocyte volume and lesser total hemocyte counts, which accounts for about 40% of that of diploid, but the hemocyte composition in triploid was the same as that in diploid. Results above provide basis and reference for stress resistance research on triploid shrimp.