The yellow pigments of Erwinia herbicola Eho 10 and of a transformed Escherichia coli LE392 pPL376 have been identified as carotenoids. HPLC separation, spectra and in some cases mass spectroscopy demonstrated the presence of phytoene (15-cis isomer), β-carotene (all-trans, 9-cis and 15-cis), β-cryptoxanthin (3-hydroxy βcarotene), zeaxanthin (3,3′-dihydroxy β-carotene) and corresponding carotenoid glycosides. In addition, lycopene and β-carotene accumulated in the presence of the inhibitor 2-(4-chlorophenylthio)-triethylamine HCl. Carotenoid content in the transformed E. coli was two-fold higher than in E. herbicola. The pattern of the carotenoids was similar in the two organisms. Inactivation of the katF gene in E. coli resulted in an 85% loweing of carotenoid formation, as did the addition of 0.5% glucose to the medium. Suppression of carotenoid formation by inactivation of the katF gene lowered, but did not abolish, the protection offered by carotenoids against inactivation by α-terthienyl plus near-ultraviolet light (320–400 nm).