Epitheliocystis disease was diagnosed in a cultured pacu (Piaractus mesopotamicus Holmberg, 1893) stock on the basis of gross pathological (greyishwhite, pinpoint-or pinhead-sized focal areas, connected strongly to the gill lamellae) and histopathological examinations (hypertrophic cells 10-40 µm in size, having a well-defined wall and a central inclusion or characteristic granular content stained with Giemsa). The disease did not occur among fishes (Cyprinus carpio L., Ctenopharyngodon idella Cuv. and Val., Sarotherodon mosambicus Peters, Hoplias malabaricus Bloch) kept together with the affected pacus. About 30-40% of the pacu stock succumbed. Since branchial lesions and heavy mortality were observed in this pacu population, the aetiological role of the epitheliocystis agent could be suspected. Based upon ultrastructural examinations this agent is considered to be a chlamydia or a chlamydia-like organism.