We performed time course, electron microscopic observation on cultured grunt fin (GF) cells that were infected with red sea bream iridovirus (RSIV) as an isolate of Megalocytivirus in the family Iridoviridae. The morphology and morphogenesis of RSIV were also observed within GF cells. Electron microscopy revealed the presence of two types of enlarged cells: inclusion body-bearing cells (IBCs), and cells allowing virus propagation within an intracytoplasmic virus assembly site and having the fragmented or absent nucleus. The IBCs contained a definite intracytoplasmic inclusion body with a virus assembly site that served for the propagation of multiple virus particles at various stages of morphogenesis. The IBCs were the same as those formed in tissues of the diseased red sea bream, which were determined to be the pathognomonic sign.