Occurrence and development of lymphocystis cells in flounder Paralichthys olivaceus, and some histochemical features are studied using light and electron microscopy and histochemical methods. Fibroblasts engulf virus particles forming phagocytosis vacuole in the cytoplasm. The infected cells proliferate and turn round and hypertrophied; basophilic inclusion bodies, Feulgen-positive, are present in the cytoplasm, and a hyaline capsule, containing acid ploysaccharide and protein, appears outside cell membrane. The mature lymphocystis cells possess numerous virus particles (200～220nm in diameter with a central core 120～140nm) and high electron-dense inclusions with budding virions at the surface in the cytoplasm, and show margination of chromatin. Moderate electron density particles (70～80nm in diameter) fill out perinuclear cisterna and arrange in crystalloid arrays in nucleus, liberated into the cytoplasm by rupture of nuclear membrane. No virus particles are observed in nucleus. Therefore, inclusion bodies are the sites of virus assembly and maturity, while replication processes of lymphocystis virus are involved in both nuclear and cytoplasmic phases. Besides, strongly basophilic link-like structures are present between adjacent cells or through connective tissue, and virus particles are observed in somewhere of the capsule, thus where may be the spread route of virus from one cell to another or to the environment. The senile lymphocystis cells release virus by the rupture of cells.