The variability of dissolved (DOC) and colloidal (COC) organic matter and of organic biochemical components and their possible relationships with heterotrophic activity have been investigated in the northern Adriatic Sea during four surveys (June 1996, February 1997, June 1997 and February 1998). Samples were collected in two frontal regions, one in the north under the direct influence of the Po River and the other in the south close to the border-line between the northern and the middle Adriatic. The overall set of data ranged from 53 to 281 µM for DOC, 44 to 98 µM for COC, 6 to 72 µM for total dissolved carbohydrates (TDCHO-C), 0.1 to 2.4 µM for free amino acids (DFAA-C), 1.2 to 9.4 µM for total dissolved amino acids (TDAA-C) and 0.01 to 7.1 µg C l-1 h-1 for bacterial carbon production (BCP). COC and TDCHO data showed tight relationships with DOC values, which were in turn negatively dependent on salinity. Organic carbon showed increases in surface waters with respect to the background DOC value of 76±10 µM averaged from bottom data in February surveys. Seasonal changes were more or less marked depending on the riverine discharge curve and related trophic dynamics in the investigated year. The more marked increases were observed in June 1996 and, based on average values, amounted to 104 and 62 µM DOC in the north and south frontal regions, respectively; in the successive June 1997, the corresponding increases were 47 and 20 µM. COC concentrations gave a high contribution to DOC and showed a large fraction of the high molecular weight class. Contrary to the increases in DOC and COC concentrations which were stronger in 1996 than 1997, mucous macroaggregates showed a much stronger occurrence in the June 1997 compared to the June 1996 survey. Considering that the aggregation of colloids is one of the main processes leading to the formation of macroaggregates, the different behaviour of DOC seasonal changes and mucilage occurrences in the two years may suggest that the qualitative characteristics of the DOC and COC pools are probably more important than their quantitative concentrations in controlling the partitioning between dissolved and aggregate matter. Bacterial production rates showed a large variability during the study period which reflected concentration gradients of inorganic and organic nutrients, differences in the lability of the organic substrate and nutrient limitations. Bioassays carried out on samples collected in different seasons and at various salinity suggested an apparent strong limitation of the bacterial activity by phosphorous