Summary.The pink pigment ofPseudomonas extorquenswas identified tentatively as an oxo‐carotenoid similar to rhodoxanthin. The production of the pigment increased at the end of the logarithmic phase of growth ofPs. extorquensin batch cultures. It was produced during growth on a wide variety of carbon and nitrogen sources, with the exception of ethanol, but not in the presence of diphenylamine, an observation which was consistent with the identification of the pigment as a carotenoid. The organism was grown in continuous culture under conditions of oxygen and magnesium limitation which might be expected to restrict the oxidation of γ‐carotenes to pink pigments. However, such limitations caused an increase in pigmentation over that of methanol‐limited cultures. Non‐pink mutants ofPs. extorquens, obtained after the use of the mutagen, ethyl methyl sulphonate, did not have growth‐rates lower than those of the parent strain. The pigment would seem to have no central metabolic role in actively growing cells but carbon is diverted to pigment production when growth is restricted by other than carbon