1. 1. 4,8,12-Trimethyltridecanoic acid has been isolated and identified from a sample of sheep perinephric fat which earlier yielded phytanic and pristanic acids. This acid had formerly been isolated in the pure state from marine fish oil 1 and whale oil 2. 2. 2. The present finding, supported by detailed studies of other investigators in this field, suggests that phytanic, pristanic and 4,8,12-trimethyltridecanoic acid have their origin in dietary phytol, and that 4,8,12-trimethyltridecanoic acid is derived by β-oxidation of pristanic acid which itself is an α-oxidation product of phytanic acid. 3. 3. In the sample of sheep perinephric fat investigated, the content of 4,8,12-trimethyltridecanoic acid was of the order of 0.1% of the total weight of fatty acids.