The haptophyte Phaeocystis antarctica Karsten is a dominant species within the seasonal bloom in the Ross Sea. One of the unique characteristics of this form is that carbon is partitioned between the cells and the colonial matrix, a relationship that is poorly documented for this region. We combined particulate organic carbon measurements and microscopic analysis of P. antarctica-dominated samples to assess the contribution of single cells, colony-associated cells, and mucilage to the carbon concentrations of waters with P. antarctica. Two cruises to the Ross Sea were completed, one in austral spring 1994 and one in summer 1995–1996. In 1994 the bloom was dominated by colonial P. antarctica that contributed up to 96% of the total autotrophic carbon, whereas in 1995–1996 a mixture of P. antarctica and diatoms occurred. P. antarctica colony volume (V ) was related to colonial cell number (NC) by the relationship V = 417 ×NC1.67. Total colony carbon (CCOL) was calculated as the sum of cell carbon (CCC) and mucus-related carbon (CM). We found the contribution of mucus carbon to be 213 ng C mm−3 of colony volume. For P. antarctica-dominated assemblages sampled at the peak of the bloom, CM represented a minor fraction (14 ± 4%) of colony carbon, and during early summer conditions CM was at most 33% of CCOL. This organism plays a cardinal role in the carbon cycle of many regions. These results constrain the partitioning of carbon between cellular material and the colony matrix, information that is necessary to accurately describe the biogeochemical cycles influenced by this species.