Analysis of the 5S ribosomal RNA from members of the eubacterial order Planctomycetales, i.e., Planctomyces, Pirella, Gemmata, and Isosphaera, reveals several unexpected features. Firstly, the primary structures are significantly shorter than those of the majority of eubacteria and vary in length between 109 and 111 nucleotides. Secondly, the lack of an insertion at position 66 is a feature not encountered before in prokaryotic 5S rRNAs. Thirdly, as compared to the proposed eubacterial "minimal" 5S rRNA structure (Erdmann and Wolters 1986) the secondary structure contains numerous base-pair transversions. The isolated position of the planctomycetes as an individual eubacterial division and the phylogenetic position of its genera are in accord with the results obtained from 16S rRNA cataloguing.