During the course of recent investigations of the carotenoid constituents of the peel of the trigeneric hybrid, Sinton citrangequat (Citrus sinensis × Poncirus trifoliata × Fortunella margarita), we isolated a small amount of a new carotenoid ketone, C33H44O, which we propose to call citranaxanthin. The visible spectra of citranaxanthin, recorded in petroleum ether and ethanol, were very similar to those of β-apo-8'-carotenal though somewhat longer in wave length, strongly indicating the presence of a conjugated carbonyl group and a decaenone chromophore similar to that of kryptocapsin. The infrared spectrum showed a band at 1662 cm.⁻¹ characteristic of a conjugated carbonyl grouping. Reduction with sodium borohydride gave a product with a hypsochromic shift. The n.m.r. spectrum exhibited singlets at τ 7.72 (end-of-chain methyl group α to a carbonyl group), 8.02 (in-chain olefinic methyl group), 8.25 (methyl group attached to C=C in the cyclohexene ring), and 8.92 (gem-dimethyl group), and a doublet at τ 2.50 (J = 16 c.P.s.) indicating the trans configuration of the double bond to which the vinyl proton β to the carbonyl is attached. On treatment with aqueous alcoholic potassium hydroxide, citranaxanthin underwent retroaldol cleavage to yield acetone (identified as its 2,4-dinitrophenylhydrazone) and β-apo-8'-carotenal. Condensation of β-apo-8'-carotenal with acetone in the presence of alcoholic potassium hydroxide afforded a compound that did not separate from natural citranaxanthin on thin layer chromatography and had identical infrared and n.m.r. spectra. These facts lead unambiguously to structure I as the structure of citranaxanthin, which is the first known naturally occurring carotenoid with a methyl ketone grouping in the side chain. Isolation involved extracting 8 kg of Sinton citrangequat peel with acetone, partitioning carotenoids between petroleum ether and 90% methanol, column chromatography on magnesium oxide-Hyflo Supercel, yielding 90 mg of citranaxanthin with m.p. 156-156°, λmax in petroleum ether 463 and 495 mp, in ethanol 475 mp, and infrared bands including 1662 cm.⁻¹ (conjugated carbonyl). Reduction with sodium borohydride gave citranaxanthol with a hypsochromic shift.