A comparative study was made of the fatty acids of several gram-positive, -variable, and -negative micrococci. The fatty acids detected in M. diversus, M. denitrificans, and M. cerificans (Acinetobacter cerificans) were similar to those previously reported in gram-negative cocci and bacilli. The major fatty acid was oleic acid and the next most abundant acids were palmitic and palmitoleic. Micrococcus agilis, a gram-positive bacterium, was shown to have a branched-chain methyltetradecanoic acid as the major component (72%). Branched-chain and odd-numbered carbon acids as well as copious amounts of palmitic and palmitoleic acids were found in M. roseus, a gram-variable organism. Correlations were made between fatty acid composition, cell wall structure, and gram reaction of M. agilis, M. roseus, and M. cerificans. Micrococcus agilis demonstrated a typical amorphous gram-positive wall, M. roseus was shown to have a layered wall, and M. cerificans had a cell wall characteristic of gram-negative bacteria. The relationship of fatty acid composition and ultrastructure to the taxonomic position of the "problematic" micrococci was discussed.