A freeze fracture morphological study of Alvinella pompejana associated epibiotic bacteria has confirmed the large variety of bacterial species already observed by scanning and thin section transmission electron microscopy. Freeze-fracture reveals striking differences in the membrane structure: The usual fracture along the hydrophobic part of the membrane for some bacteria, but only an unusual cross-fracture through the membrane for others. This suggests that some bacteria contain non-fracturable lipids typical of certain archaebacteria.