The aerobic degradation of phytoplankton debris collected in Dutch coastal waters on 2 days in 1991 (15 April and 8 May), representing 2 physiological stages of a phytoplankton spring bloom dominated by Phaeocystis sp., was studied in batch culture experiments. The bacterial production and the concentrations of particulate organic carbon (POC) and dissolved organic carbon (DOG) were monitored over a period of 102 d. Bacterial numbers and biomass were followed for 35 d. All experiments showed a rapid metabolic response of bacteria and a sharp decrease in the concentration of POC and DOC during the first days of the experiments. Thereafter bacterial production rates remained constant, and POC and DOC decreased slowly. Apparently, the phytoplankton debris consisted of a labile, rapidly degradable fraction and a refractory, slowly degradable fraction. The labile fraction comprised approximately 50 % of the debris, and was degraded with a bacterial carbon conversion efficiency of between 10 and 20%. There were no indications that antibiotic compounds present in the algal debris inhibited the degradation. Acrylate, a proposed antibiotic compound which was present in the algal debris, was rapidly degraded in a control experiment. The percentage of the material that had been degraded after 102 d was highest in the experiment with material collected in May. It was concluded that during the early phase of the bloom, more refractory compounds are produced.