ABSTRACTThe new genusPycnococcusGuillard is based on several clones from the western North Atlantic and Gulf of Mexico. The type and only described species,Pycnococcus provasoliiGuillard, sp. nov., is typified by cloneΩ48‐23 from the North Atlantic. Cells ofPycnococcus provasoliiare solitary, spherical, 1.5–4.0 μm in diameter, have a resistant cell wall lacking sporopollenin, and have the ultrastructural characteristics of green algae. With the light microscope they are scarcely distinguishable from cells of other coccoid planktonic organisms. In pigmentationP. provasoliiresemblesMicromonas pusilla, Mantoniella squamata, andMamiella gilvain having chla, much chlb, Mg 2,4‐divinylphaeoporphyrin a5monomethyl ester (presumably), and prasinoxanthin as a major xanthophyll. The pyrenoid ofP. provasoliihas a cytoplasmic channel, which is unique among species closely related to it. Flagellates, occurring rarely in culture, are similar to but distinguishable from knownPedinomonasspecies by size and shape.Pycnococcus provasoliiis referred to the new family Pycnococcaceae Guillard, in the order Mamiellales of the class Micromonadophyceae (Chlorophyta). Clones ofPycnococcus provasoliiare oceanic in nutritional characteristics, require only vitamin B12in culture, and are well adapted to growth under blue or blue‐violet light of low