Ultrastrucural and molecular data were used to examine a new alga (CCMP2000) collected from the Arabian Sea. Non-motile vegetative cells are typically oblong to ovate, solitary and lack a gelatinous envelope. Ultrastructural analyses of these cells revealed the presence of a nucleus located in the posterior portion of the cell. A single large Golgi body with numerous cisternae is located anterior to the nucleus as are numerous profiles of mitochondria possessing tubular cristae. Two peripheral plastids are present having immersed pyrenoids that are traversed by one or more thylakoids. In some cells the plastids are conspicuously lobed forming finger-like extensions. Non-motile vegetative cells possess reduced mature and immature flagella as well as a haptonema that are 1 micron long or less. A distinct internal covering and external wall surround the cell protoplasm. The outer wall is apparently composed of simple scales, but the composition (organic or mineralized?) of these scales is unknown. The outer wall may form a protuberance, or "foot", at the posterior end of non-motile cells. Interestingly, many vegetative cells are infected by a labyrinthulid. Flagellate stages have been observed and possess roughly equal length flagella and a haptonema. Phylogenetic analyses of nuclear 18S rRNA and plastid-encoded rbcL genes indicate CCMP2000 is most closely related to Phaeocystis. However, CCMP2000 does not fit the description of Phaeocystis and therefore probably warrants recognition as a new taxon.

Key words: 18S rRNA, algae, Phaeocystis, Prymnesiophyceae, rbcL, systematics