Abstract The contribution of capsule-bearing bacteria to the total number of bacterioplankton community was enumerated to assume the metabolically active number of bacterioplankton in the eight different water masses of the Marmara Sea, Kucukcekmece lagoon and Istanbul strait. Capsulated and non-capsulated bacteria were determined using modified the negative staining technique. Total bacteria abundance and capsulated bacteria number was compared according to sampling area. As a result capsulated bacteria number was found higher at the surface water of eutrophic lagoon to the total bacteria (25.8 %) as compared to the water column of the Northern Marmara Sea and Istanbul Strait. Capsulated bacteria 5-20 meter, was found less than lagoon samples. The surface water of lagoon percentage of capsulated bacteria was significantly higher (P<o.oo1) of="" was="" and="" significant="" in="" there="" were="" which="" the="" that="" than="" <="" p="" all="" areas.="" it="" observed="" no="" statistically="" differenve="" between="" dates="" sampling="" respect="" to="" contribution="" number="" samples,="" taken="" water="" above="" sandy="" sediments="" marmara="" sea="" deep="" from="" capsulated="" bacteria="" bacterioplankton="" community.="">