1. The growth factor from Mycobacterium phlei is now named mycobactin P to distinguish it from related but chemically distinct growth factors from other species of mycobacteria. It is shown to comprise four closely similar chemical entities differing only in the fatty side chain. 2. The structure of the main component (85%) of mycobactin P has been established and the absolute configuration of all asymmetric centres determined. 3. Ferric mycobactin P has been crystallized; its structure is discussed. 4. Evidence is given for regarding ferric mycobactin P as a member of the class of compounds known as sideramines, though the mycobactins are more specialized than other sideramines.