Morphological characteristics of neutrophil in common carp, Cyprinus carpio infected with pathogenic bacterium Aeromonas hydrophila were examined by light microscopy. The neutrophil was round or oval, and larger than the neutrophil of uninfected carp. The nucleus was round to oval. The neutrophil possessed highly developed Yasumoto body (Y-body). The Y-body formed mesh work structure and basophilic staining with May-Grünwald Giemsa. The Y-body was stained with toluidine blue or hematoxylin, but not with periodic acid Schiff reaction, alcian blue, alkaline phosphatase, acid phosphatase, β-glucronidase, α-naphtyl acetate esterase, α-naphtyl butyrate esterase, naphthol AS-D chloroacetate esterase and peroxidase.