The lipids of Treponema hyodysenteriae B78, the etiologic agent of swine dysentery, comprised 16.4% of the cell dry weight, and consisted of 37.4% glycolipids, 28.6% phospholipids, and 34.0% neutral lipids. Monogalactosyldiacylglycerol, a major lipid in all Treponema except Treponema pallidum, comprised 80% of the glycolipids. An unidentified galactolipid less polar than monogalactosyldiacylglycerol was also detected. Phosphatidylglycerol (19.5% of the total lipids) was the major phospholipid. Phosphatidylcholine, characteristically the major phospholipid of treponemes, comprised 6.1% of the total lipids. Cardiolipin and lysophosphatidylcholine were minor components. The alk-1-enyl ether forms of both the phospholipids (plasmalogens) and glycolipids predominated. The alk-1-enyl ether forms of monogalactosyldiacylglycerol, the unidentified galactolipid, phosphatidylglycerol, cardiolipin, and phosphatidylcholine were 88.3, 96.4, 74.8, 60.6, and 6.3%, respectively. The acyl and alk-1-enyl chains of the organism were qualitatively similar and differed dramatically from those of the medium indicating a capability for fatty acid synthesis that most Treponema do not possess. Saturated C14, C15, and C16 chains comprised more than 95% of the acyl and alk-1-enyl groups. About 25% of the chains were iso-15:0, anteiso-15:0, and other branched moieties.