The nitrite-oxidizing autotroph, Nitrobacter was studied with respect to fatty acid composition and lipids. One fatty acid, shown to be cis-11-actadecenoic acid ( cis-vaccenic) accounted for almost 96% of the total fatty acids of the extractable lipids of Nitrobacter agilis, Nitrobacter winogradskyi and each of several isolates from Minnesota and Moroccan soils studied. The cis-vaccenic acid was high in all organisms, ranging from 85 to 95% when grown at 27°C in the log growth phase, the other major acid was palmitic (16 : 1). All-converted cis-vaccenic acid to a 19-carbon cyclopropanecarboxylic acid upon entering the stationary growth phase. The 11-carbon cyclopropanecarboxylic acid was not degraded when stationary phase cells were re inoculated into fresh medium. In N. agilis the levels of cis-vaccenic acid ranged from 86.8% when grown at 33°C to 95.6% when grown at 19°C. Addition of acetate or casein hydrolyzate to the inorganic medium had virtually no effect on the fatty acid composition of N. agilis, while propionate effected both qualitative and quantitative changes. In all organisms phosphatidylcholine made up a large portion of the extractable lipids. The distribution was phosphatidylcholine, 54%; phosphatidylethanolamine, 23%; phosphatidylglycerol, 10%; and neutral lipids, 11% for N. agilis.