THE taxonomic group Prochlorales (Lewin 1977) Burger-Wiersma, Stal and Mur 1989 was established to accommodate a set of prokaryotic oxygenic phototrophs which, like plant, green algal and euglenoid chloroplasts, contain chlorophyll b instead of phycobiliproteins. Prochlorophytes were originally proposed (with concomitant scepticism1-3) to be a monophyletic group sharing a common ancestry with these 'green' chloroplasts4-6. Results from molecular sequence phytogenies, however, have suggested that Prochlorothrix hollandica 7 is not on a lineage that leads to plastids8-12. Our results from 16S ribosomal RNA sequence comparisons, which include new sequences from the marine picoplankter Prochlorococcus mariniis13,14 and theLissoclinum patella symbiont Prochloron sp.15, indicate that prochlorophytes are polyphyletic within the cyanobacterial radiation, and suggest that none of the known species is specifically related to chloroplasts. This implies that the three prochlorophytes and the green chloro-plast ancestor acquired chlorophyll b and its associated structural proteins in convergent evolutionary events. We report further that the 16S rRNA gene sequence from Prochlorococcus is very similar to those of open ocean Synechococcus strains (marine cluster A (ref. 16)), and to a family of 16S rRNA genes shotgun-cloned from plankton in the north Atlantic and Pacific Oceans17-19.