In this study, the histological structure of the hepatopancreas of Mytilus gallolprovincialis Lamarck, 1819 was examined. The samples were collected at Yenikapi shores in the Northern part of the Marmara Sea between June 2004-June 2005. The samples collected were prepared for light microscopic examinations and stained by the h.istological stains. Lipid as an energy reserve and activity of catalase related with the detoxification function were evaluated seasonally. It was demostrated that under the influence of environmental and biotic factors such as gonadal alterations and food availability, lipid reserves and activity of catalase shows seasonal alterations.