The population of the centrally located rhabdomeres no. 7 in the ommatidia of flies (Musca, Calliphora, Drosophila) is inhomogeneous: approximately 2/3 of them contain — besides a photoisomerizable rhodopsin — a photostable pigment. Its extinction spectrum has a maximum at 460 nm and two shoulders at 430 and 485 nm respectively. Extinction is maximal for theE-vector perpendicular to the microvilli. Whereas the functional role of the photostable pigment for receptor 7 has still to be worked out, its functional consequence for receptors no. 8, which are located proximal to receptors 7, is obvious: it modifies their spectral sensitivity by selectively absorbing blue light. Due to this “screening”-effect, a shift of the maximal sensitivity of receptors no. 8 is predicted from 490 nm (maximal sensitivity of unscreened receptor 8, Harris et al., 1976) to 520 to 540 nm. This is in agreement with recent electrophysiological data (Hardie, 1977). The results show that spectral sensitivities of receptors no. 8, as determined by means of the ERG of white-eyed mutants or of mutants lacking receptor 7, do not represent the spectral sensitivities of most of these receptors in wild-type flies.