18S rDNA sequences were obtained for the prasinophytes Pseudoscourfieldia marina (Throndsen) Manton and Pycnococcus provasolii Guillard. These sequences, along with those of additional prasinophytes and other green algae and land plants, were used for phylogenetic analyses by the neighbor-joining, maximum parsimony, and quartet puzzling methods. Results indicate that Ps. marina and Py. provasolii are closely allied and that both should be included in the Pycnococcaceae. In addition, results of these sequence analyses and additional pigment analysis indicate that the organism previously identified as Ps. marina, isolate CCMP 717, is, in fact, a Nephroselmis sp. or is closely related to that genus.