Distribution of heterotrophic bacteria was studied during spring period in 2004 at two different locations in the Gulf of Trieste (Adriatic Sea). Bacterial abundance was determined using the counting method on epifluorescence microscope. Bacterial production was measured with the method of incorporation of radio-labelled 3Hthymidine and 3H-leucine. Bacterial abundance at both locations varied between 0,2z109 and 2,1z109 cell/L. In May the bacterial biomass was low, but an increase in biomass quantity in the period of mucus macroaggregate appearance was observed. In further studies themucus macroaggregate samples were examined with different microscopic (optical, epifluorescence and electronic) and microbiological techniques. Bacterial colonies isolated on agar plates formed different morphological details. Preliminary analysis of bacterial phylogenetic diversity was performed with the fluorescence in situ hybridisation (FISH) method. The influence of addition of inorganic and organic nutrient on bacterial growth dynamics were studied on previously isolated bacteria in two laboratory experiments. The concentration of nutrient, total carbohydrates and protein were studied simultaneously with bacterial biomass and production. Mucus with high portion of polysaccharides served as an organic nutrient for bacteria. Asconsequence changes in carbohydrates concentration were observed probably duo to increased bacterial enzymatic activity, and in the same time bacterial consumption of released carbohydrates. Our preliminary results showed that therole of heterotrophic bacterial community is of great significance in the process of transformation of organic matter in marine ecosystem.