Ten moribund fish were received at pathology laboratory to establish the cause of sponteneous mortalities of farmed tilapia recorded in Kilosa District, Morogoro region. Post-mortem examination revealed macroscopic mucous bands connecting gills and operculum; and oval to round grey-white cysts on the gill lamella. Gill samples were collected for microscopic and bacteriological investigation. Histopathological investigation revealed gill epithelial hyperplasia and characteristic enlargement of epithelial cells infected with pale and basophilic cytoplasmic inclusions. The gross pathological changes coupled with histopathological findings recorded in this case are typical features of epitheliocystis. Furthermore, fish mortalities ceased in the farm after water replacement and reconnection of all fish ponds with direct supply of fresh water from the source. This is the first report in Tanzania describing characteristic epitheliocystis lesions. Furthermore, this report re-affirm previous findings that epitheliocystis can be managed through routine management of water quality.