AbstractThe prasinophycean algaMantoniellacontains, in addition to Chl a and b, at least a third green pigment which is functionally active in the light‐harvesting antenna. This third Chl was isolated in order to elucidate its chemical structure. The absorption and fluorescence spectra were measured not only from the purified pigment but also from its pheophytin and its methylpheophorbide. The spectra were compared with those of authentic Chl c‐1 and c‐2, which were isolated from the diatomNitzschiasp. and with Mg‐DVPP (purified fromRhodobacter).The results show that the pigment fromMantoniellacompares best with Chl c‐1. In order to clarify the spectral data, Chl c‐1 and c‐2, Mg‐DVPP, and the pigment fromMantoniellawere subjected to a chromatographic system that is able to separate these porphyrins. The chromatographic analysis clearly shows that the pigment fromMantoniellaco‐migrates with Chl c‐1 and not with the bacterial pigment.Mantoniellais the first organism which has been demonstrated to contain Chl a,