Pyramimonas Schmarda is a genus of unicellular green flagellates, recorded in marine water and sea ice samples. Pyramimonas is within the prey size range of the most important protozoan grazers in Disko Bay, West Greenland, where this study took place. Despite the potential ecological importance, little is known about the occurrence of the genus. The aim of this study was to explore the biomass of Pyramimonas in developing stages of sea ice and in the water column. Pyramimonas colonized the early stages of sea ice, and the highest percent of Pyramimonas biomass was found in grease ice. The biomass of Pyramimonas was more than a magnitude higher within sea ice compared to the surface water. The results illustrate that Pyramimonas from the ice is an important contributor to the plankton community prior to the spring bloom. An undescribed species, Pyramimonas diskoicola sp. nov., was found. Based on morphology and ultrastructure, combined with molecular phylogeny inferred from the small-subunit SSU rDNA and the large-subunit chloroplast-encoded rbcL, the species was placed in subgenus Vestigifera. The cells possessed four flagella, measured 8.3 +/- A 2.6 mu m in length and 5.1 +/- A 0.8 mu m in width, and were characterized by an uplifted quadrant in the center of the box scales, not seen at any other Pyramimonas species. The phylogenetic analyses indicated P. diskoicola to be closely related to other polar sea ice species of Pyramimonas.