An obligately anaerobic bacterium that lacked a cell wall was isolated from the hypolimnion of Ace Lake, Antarctica. Cells were very pleomorphic, forming cocci, filaments up to 25 micrometres in length, and annular shapes. The organism was morphologically very similar to some members of the class Mollicutes which contains two genera of obligately anaerobic bacteria, Anaeroplasma and ... Asteroleplasma. Like members of the class, the isolate was resistant to high concentrations of penicillin (1000 Units per millilitre). Similar to Anaeroplasma, the organism had a low DNA G+C content (29.3 +/- 0.4) and produced hydrogen, carbon dioxide, acetic acid, lactic acid and succinic acid from the fermentation of glucose. However, the taxonomic status of the strain remained unclear as, unlike members of the class Mollicutes, the isolate had a relatively large genome size (2.26 +/- 0.11 x 10 to the 9 daltons), did not pass through 0.45 micrometre pore size filters, and did not form typical mycoplasma-like colonies. The organism was psychrophilic with an optimum temperature for growth between 12 degrees C and 13 degrees C. A phenotypic description of the organism is given and the ecological role of the organism is inferred from its phenotype and the characteristics of its Antarctic habitat.