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A one-year survey of HNA and LNA heterotrophic bacteria abundance and its relation to bacterial production was conducted in the middle and southern Adriatic. The average abundance of heterotrophic bacteria in the central and southern coastal areas of the eastern coast of the Adriatic Sea ranged from 0.24x106 to 2.39x106 cells mL-1 and in the open sea area ranged from 0.23x106 to 0.63x106 cells mL-1. Elevated average abundances were found in the coastal area of Sibenik and in Kastela Bay. The average monthly percentage of HNA bacteria in the central and southern coastal areas investigated ranged from 21.48 to 93.18%, while in the open sea HNA ranged from 28.62 to 65.62%. The seasonal distribution in the coastal areas mostly showed simultaneously the prevalence of LNA bacteria in the bacterial community and high values of bacterial production, respectively, during spring and summer. The prevalence of the HNA group in the bacterial community was found during colder seasons together with low values of bacterial production. Exceptions are identified in the areas influenced by rivers with the highest bacterial production, where HNA bacteria were dominant in the microbial community during all four seasons. On the open sea during the periods with the highest values of bacterial production, when viewed seasonally and vertically, the water column was dominated by LNA bacteria indicating the importance of both groups of bacteria in the marine ecosystems of the waters studied. [ABSTRACT FROM AUTHOR]