The phylogeny of an Antarctic, cell wall-less, bacterial strain was determined by sequencing PCR amplified 16S rDNA, and comparison of the sequence with other bacterial 16S rRNA sequences available in databanks. Although the strain was phenotypically very similar to members of the genus Anaeroplasma, phylogenetic analyses showed it was a member of the order Spirochaetales. Until now, the order was one of the few bacterial orders in which phylogeny was reflected in a uniform morphology of its members. The viability of wall-less cells in cultures of spirochetes and spirochetal infective material warrants reinvestigation.