Distribution of phyto-and bacterioplankton growth and biomass parameters were followed during a spring bloom in the marine Oosterschelde basin, Netherlands. Several other biotic and abiotic parameters were measured. During the measuring period from 4 April until 4 June, concentrations of the dissolved nutrients silicate, ammonium, and nitrate decreased, while dissolved phosphate tehded to increase. Concentrations of chlorophyll a increased with a distinct maximum in mid May during a bloom of the haptophycean Phaeocystispouchetii. In the shallower part of the basin maxima of particulate primary production and bacterioplankton production were reached at the end of A p r~l during a mixed bloom of diatoms of the genera Cryptornonas, Skeletonerna and Thalassiosira as well as small flagellates. In the central basin, the most pronounced particulate primary production was observed during the P. pouchetii bloom. Relatively low percentages of extracellular release of total primary production were observed during and after this bloom. Percentage of bacterial incorporation of total exudates was high at the beginning of the experiment and decreased until the P. pouchetii bloom, when it increased again. Maximum bacterial productivities and biomasses were recorded during the decline of the P. pouchetii bloom. Overall oxygen consumption rates seemed to be determined by phytoplankton biomass and by bacterioplankton production, but not by phytoplankton production or bacterioplankton biomass. Concentrations of dissolved free amino acids (DFAA) tended to decrease until the end of the large P. pouchetii bloom when an increase was observed; lowest concentrations were durlng this bloom. Alanine, aspartate, glycine, leucine and serine were always quantitatively the most important amino aclds; during the P. pouchetii bloom also glutamate. Mole percentages of aspartate and methionine increased at the end of this bloom.