The lipids of Treponema innocens, type strain B256, formerly considered a nonpathogenic isolate of T. hyodysenteriae, have been analyzed and compared with the lipids of T. hyodysenteriae. The lipids of T. innocens comprised 16% of the cell dry weight. Polar lipids amounted to about two-thirds of the total lipids and consisted of 61.9% phospholipids and 38.1% glycolipid. Neutral lipids consisted mainly of sterols. The phospholipids were principally phosphatidylglycerol, phosphatidylcholine, and cardiolipin. Minor amounts of lysophosphatidylcholine, sphingomoyelin, and a relatively nonpolar, unidentified phospholipid were present. The latter lipid has not been detected in T. hyodysenteriae. The glycolipid fraction of T. innocens contained a single component, monoglucosyldiglyceride, in contrast to the occurrence in T. hyodysenteriae of two components: monogallactosyldiglyceride and a less-polar glycolipid tentatively identified as acylmonogalactosyldiglyceride (the additional acyl moieties being 86.6% acetyl, 11.6% propionyl, and 1.6% n-butyryl groups). Alk-1-enyl ether analogs comprised 24.6% of the total phospholipids and glycolipid of T. innocens, or about one-third of the amount in T. hyodysenteriae. The acyl and alk-1-enyl moieties of T. innocens consisted of greater than or equal to 92% of 14:0, iso-15:0, and 16:0 chains. In contrast to T. hyodysenteriae, anteiso-15:0 moieties were not detected, and a reversed distribution of 14:0 and iso-15:0 alk-1-enyl moieties occurred in the two species.