Cellular fatty acid composition of 50 strains of the genus Pseudomonas was determined by gas-liquid chromatography. Straight-chain saturated acid of C_16:0 and straight-chain unsaturated acids of C_16:1 and C_18:1 with a double bond were commonly found in all the strains tested. The presence of hydroxy acids, cyclopropane acids, and branched-chain acids showed the characteristics for the groups and species in the genus Pseudomonas. Similarity values calculated on the basis of fatty acids exhibited a clear correlation to the taxonomic groups of this genus. Bacterial fatty acid composition was considered to be useful for the study of interrelation and for rapid identification of the bacteria. Taxonomic studies on bacteria have been mainly carried out on the basis of morphological, biochemical, and physiological characteristics. Recently, chemical constituents of bacterial cells have become of interest from the point of taxonomy, and DNA base composition, cell wall composition, and type of co-enzyme Q have been used for taxonomic criteria in some taxa. The progress of instrumental analyses has contributed to the development of chemotaxonomy, and the results involved have been useful not only for the bacterial classification, but also for rapid identification of the bacteria.