A universal phylogenetic tree of organisms from all kingdoms was constructed by the use of elongation factor Tu (EF-Tu) as the marker molecule. As in the 16S ribosomal RNA (16S rRNA)-based phylogeny, the EF-Tu tree divides eukaryotes, archaebacteria and prokaryotes into three main branches. Furthermore, the EF-Tu-based tree shows, in contrast to the 16S rRNA tree, some interesting evolutionary relationships between mycoplasmas, better reflecting phenotypic features of these organisms.