Compound eyes from adult flies of the ommochrome deficient mutant yellow of the Australian sheep blowfly, Lucilia cuprina, are processed for transmission electron microscopy using conventional methods. The ultrastructure of the pigment granules in the primary and secondary pigment cells is compared with that in corresponding cells of the wild-type strain and of yellow adults raised from larvae fed a diet supplemented with 3-hydroxykynurenine (3-HK). The pigment granules in both cell types of yellow are shown to be of the "empty" type in contrast to those in the wild-type strain. The pigment granules in eyes of yellow adults raised from larvae fed 3-HK are effectively normalized. The results are discussed in terms of previous electron microscope studies on other dipteran eye-colour mutants.