Ribosomal large and small subunits are organized in four general structural patterns. The four types are found in ribosomes from eubacteria, archaebacteria, eukaryotes, and a group of sulfur-dependent bacteria ( eocytes ), respectively. All four ribosomal types share a common structural core, but each type also has additional independent structural features. The independent features include the eukaryotic lobes and the archaebacterial bill on the smaller subunit. On the larger subunit, they include the eocytic lobe, eocytic gap, and eocytic bulge and a modified central protuberance. These data are most parsimoniously fit by a single unrooted evolutionary tree. In this tree eocytes are closely related to eukaryotes, while archaebacteria and eubacteria are closest neighbors. The tree is consistent with currently known molecular biological properties and indicates that eocytes have a phylogenetic importance equal to that of the three known kingdoms. When other properties and molecular mechanisms of these organisms are better defined, we suggest that an appropriate kingdom name for this group would be the Eocyta .