Mucus from the microalga Phaeocystis was partially purified and used as the C source in various stable enrichment cultures wherein the mucus carbohydrates were degraded under oxic as well as anoxic conditions. The breakdown of mucopolysaccharides (initially at a rate of 50% in 11 d at 12 degrees C) markedly slowed after about 15 d, leaving a fraction nondegraded. The carbohydrate composition of the residual fraction did not change during breakdown. It was shown that the incomplete degradation of mucopolysaccharides was not due to inherent resistance to breakdown in parts of the mucus carbohydrates but to the release of inhibitors produced during breakdown. The (potentially high) overall degradability of mucopolysaccharides has important consequences for the fate of C fixed by Phaeocystis. The balance between recycling of Phaeocystis biomass in surface waters and its accumulation, sedimentation, and burial in the sediment will greatly depend on factors such as nutrient availability, presence of inhibiting agents, and composition of the microbial community, rather than on structural impediments inhibiting mucopolysaccharide degradation.