Microbial degradation of Phaeocystis globosa colonies and their derived organic matter by free-living and attached bacteria was investigated in Belgian coastal waters during the spring development of diatom- Phaeocystis colonies in 1994. Results obtained show concomitant evolution of hydrolytic ectoprotease and b-ectoglucosidase ectoenzymatic activities with respect to the phytoplankton bloom, suggesting that the low biodegradability of Phaeocystis colonies leading to transient accumulations of Phaeocystis -derived material in the coastal North Sea was not due to a lag phase required for the induction of b-ectoglucosidase. Up to 66% of total bacterial biomass was found attached to particles larger than 10 μm. While occurring always in low abundance compared to free-living bacteria, both the average specific biomass and growth rate of particle-attached bacteria were very high, i.e. 60 fg C cell -1 and 0.28 h -1 , respectively. Similarly, specific ectoenzymatic activities of particle-attached bacteria were on average about 5 times higher than those characterising free-living bacteria. Budget calculations show a 53% contribution of Phaeocystis -attached bacteria to the mineralisation of Phaeocystis -associated production, i.e. a 53:47% role for attached and free-living bacteria, respectively.