The genera of sixty-three bacterial strains isolated from bovine rumen contents were determined according to the identification schemes of Hungate, and Bergey's Manual of Determinative Bacteriology (8th Ed.). Then, the cellular fatty acid and aldehyde composition of all these organisms was analyzed by gas-liquid chromatography, gas chromatography-mass spectrometry, and mass fragmentography. The distinctive features of these organisms were presented. Succinivibrio contained 18: 1 (w-7), 16: 0, 14:0,16: 1, and 3-OH-14: 0 as the most abundant fatty acids. Lachnospira contained 18: 0, 16: 0, and 18: 1 (w-7) as the most abundant acids. Anteiso-l5: 0 and 15: 0 were the most abundant acids in Bacteroides. Iso-16: 0, anteiso-17: 0, 16: 0, and 18: 0 were the most abundant acids in an unidentified gram-negative obligately anaerobic rod. The Butyrivibrio strains were divided into two major groups based on fatty acid and fatty aldehyde composition. One group contained branched-chain fatty acids and fatty aldehydes as major components, and the other had straight-chain ones. The former group contained anteiso-15: 0 fatty acid (FA) and anteiso-15: 0 aldehyde (ALD) as major components. In the latter group, a considerable variation was found in the relative amount of each fatty acid and fatty aldehyde. The major fatty components in Selenomonas were 16:1 FA, 14: 0 FA, 3-OH-14: 0 FA, and 16: 1 ALD. Lactobacillus contained 16: 0 FA, 16: 0 ALD, 16: 1 ALD, and 16: 1 FA as major components, and in Bifidobacterium they were 18: 1 (w-9) FA, 16: 0 FA, 14: 0 FA, and 18: 1 ALD. The major fatty acids of Propionibacterium were iso-15:0,16: 0, and iso-17: 0, and those of unidentified gram-positive chainforming anaerobic coccus were anteiso-15: 0 and anteiso-17: 0. Each of the nine different kinds of bacteria mentioned above, but not Butyrivibrio, formed a single dendrogram cluster according to their fatty acid and fatty aldehyde composition. The Butyrivibrio species were divided into three clusters.