Mevalonic acid provided to detached tomato fruits is the most efficient precursor of carotenoids yet reported. Much of the mevalonic acid is incorporated into a series of nonsaponifiable colorless compounds with chromatographic behavior the same as the known carotenes.Three carotenes, phytoene, β-carotene, and lycopene, which constitute about 88% of the carotenes of tomato fruit, have been freed from these contaminants, and ratios of incorporation of various substrates into carotenes and total nonsaponifiable materials can be calculated. Acetate is apparently incorporated via mevalonic acid. Glucose is a more efficient precursor of carotenes than acetate though less efficient than mevalonic acid, but only small amounts are incorporated into the unknown colorless compounds. Carbon dioxide is incorporated in about the same ratio as glucose.Glucose and carbon dioxide appear to be incorporated into carotenes through a 20-carbon precursor (3,7,11,15-tetramethylhexadeca-1,3,6,10,14-pentaene) although the turnover rate is much faster than with mevalonic acid.It is suggested that the differences in the incorporation of mevalonic acid and glucose may be due to different sites of carotene synthesis within the cell.