WindartiPutra, Ridwan MandaArdiansyah, Muhammad2016-04-022016-04-022016-04-02978-979-792-437-9wahyu sari yenihttp://repository.unri.ac.id/xmlui/handle/123456789/8155Ompok hypopthalmus (Siluridae) is commonly inhabit the Siak River. This river is relatively polluted as there are human activities present in the riverside along the river and the pollutant produced flown to the river. Due to pullutant accumulation, water quality in the downstream of the river may be worse than that of the upstream. As water quality affects the health status of fish, the health condition of fish living in the upstream and downstream of the river might be different and it may be reflected in the histological structure of main organs of the fish. To understand the difference of histological structure of the gill, kidney and liver of O. hypopthalmus, a study has been conducted. Fish samples were captured from the upstream and downstream of the river. Gill, kidney and liver tissue was processed for histological study (formalin fixed, alcohol series processed and HE stained). Results indicate that the histological structure of the fish from both sampling areas showing abnormality symptoms. In the gill tissue of fish from both areas, most of secondary lamella are fused and abnormality symptoms such as hyperplasia, hypertrophy and necrosis are present. In the kidney and liver tissues, there are hemorrhage, mineralization and cell degeneration. The kidney and liver of fish from the downstream shown more mineralization spots than that of the upstream. These data suggested that the fish living in the upstream and downstream of the Siak River have been exposed to relatively polluted water that negatively affects their health in generalenOmpok hypopthalmusupstream and downstreamSiak Rivertissue structureRiauUR-Proceedings