Abstract.In the water‐column of the Gulf of Trieste (Northern Adriatic Sea) flocculent, amorphous aggregates known as marine snow reach maximum density and dimensions up to 20cm in diameter during summer months. Sampling of these fragile macro‐aggregates was carried out by SCUBA‐divers.A large detrital component was found to be embedded in an organic matrix together with a large array of intact phytoplankton species. Heterotrophic bacterial density ranged from 2–65 × 108cells‐g‐1(marine snow dry wt) and is therefore of the same order of magnitude as in the surface layer of the sediments of the Gulf. In terms of biomass, bacteria reached only 30–95% of heterotrophic microflagellates.A mean abundance of marine snow of 5g (marine snow dry wt) m‐3was obtained for the water‐column above the pyenolinc. At this boundary layer, however, a mean abundance of about 10g (marine snow dry wt) m‐3led to the formation of a distinct, 10‐50cm thick layer. Based on these data and sedimentation rates of marine snow to the pyenoclinc (38 g [marine snow dry wt] m‐2d‐1) the potential ecological significance of these macro‐aggregates for the pelagic system of the