Fatty acid compositions of phospholipids from Selenomonas ruminantium cells, a strictly anaerobic bacterium that requires a volatile fatty acid for normal growth in a glucose medium, were examined. Under fatty acid- deficient conditions, the cells appear to utilize propionate, a normal fermentation product from glucose, as a precursor of odd-numbered long-chain fatty acid moieties of phospholipids. When labeled 1-¹⁴C-butyrate or 1-¹⁴C- caproate was supplemented to the growth medium, even-numbered fatty acids in phospholipids were all radioactive. The odd-numbered acids, however, carried no significant radioactivity. When cells were grown with 1-¹⁴C-valerate, labeled odd-numbered and unlabeled even-numbered fatty acids were detected as components of phospholipids. This phenomenon appears to be due at least in part to the occurrence of α-oxidation of added volatile fatty acids, since imidazole, which is known to be an inhibitor of α-oxidation, inhibited the formation of even-numbered long-chain fatty acids in cells grown in valerate and heptanoate. Imidazole at 30mM did not inhibit cell growth with even-numbered fatty acids (C₄ and C₈), but growth with odd-numbered fatty acids (C₅ and C₇) was inhibited significantly. The growth inhibition by imidazole was partially released by the addition of a very small amount of butyrate or caproate. It was concluded that even-numbered fatty acids are important constituents of phospholipids for normal growth of this organism.