The synthesis of eye pigments has been studied in the seven eye color mutants of the Australian sheep blowfly, Lucilia cuprina. Six appears to be affected primarily in the synthesis of xanthommatin. In wild type, the onset of xanthommatin biosynthesis occurs midway through metamorphosis. Developmental patterns of accumulation of the xanthommatin precursors tryptophan, kynurenine, and 3-hydroxykynurenine have also been established for wild type. By determining the levels of these precursors in late pupae of the mutants, it has been shown that the mutant yellowish accumulates excess tryptophan and the mutant yellow accumulates excess kynurenine. The implications of these results--that yellowish lacks tryptophan oxygenase, thus failing to convert tryptophan to kynurenine, and that yellow lacks kynurenine hydroxylase (blockade in the conversion of kynurenine to 3-hydroxykynurenine)--have been confirmed. This has involved in vitro assays of tryphophan oxygenase and precursor feeding experiments. The precursor accumulation patterns are less clear for the other mutants.