THE branched-chain fatty acid 3,7,11,15-tetramethyl-hexadecanoic acid (phytanic acid) has been isolated and identified in butterfat1–4, sheep fat5, ox fat6, ox serum7,8, crude petroleum9 and also in the tissues, serum and urine of humans afflicted with the rare disease referred to as Refsum's syndrome10–12 (heredopathia atactica polyneuritiformis). This communication reports the occurrence of trace quantities of this acid in the rumen bacteria from a fistulated cow fed clover-grass hay.