The main carotenoid of Flavobacterium strain R1560 has been identified as (3R,3'R)-zeaxanthin. Also present were small amounts of 15-cis-phytoene, phytofluene, "zeta-carotene" (7,8,7',8'-tetrahydro-psi, psi-carotene plus 7,8,11,12-tetrahydro-psi, psi-carotene), neurosporene, lycopene, beta-zeacarotene, gamma-carotene, beta-carotene, beta-cryptoxanthin, rubixanthin, 3-hydroxy-beta-zeacarotene and several apo-carotenals. Zeaxanthin production was inhibited by nicotine (10 mM), and lycopene and rubixanthin accumulated. The biosynthesis of zeaxanthin is discussed in terms of pathways and also of "half-molecule" reaction sequences. The presence of zeaxanthin may be a characteristic of a group of Flavobacterium species, and may thus be useful in the taxonomic classification of these organisms.