Tissue reactions in gills of cultured red seabream, Pagrus major, toan epitheliocystis infection are described. Basophilic intracellular inclusions in gills contained prokaryotes, most probably a Chlamydia-like organisms according to morphological characteristic. A few types of tissue reaction were found around the inclusions: encapsulation, epithelial hyperplasia, lamellar fusion, and inflammation. It was considered that eosinophilic granule cells and macrophages might take part in defense reactions against this prokaryotic organism