THE use of behavioural mutants of Drosophila to elucidate functions of the nervous system has produced several nonphototactic mutants snowing abnormality in the recorded waveform of the electroretinogram (ERG)^1,2. We are working with a mutant strain of D. melanogaster which, though behaving phototactically positive in a T-maze under low ambient light, is visually impaired and behaves as though blind in a simple optomotor apparatus where normal, wild-type flies will orientate to visual cues^3,4. The mutant appeared in a highly inbred line and is indistinguishable from the wild-type in external appearance. Detailed mapping is in progress but, at this stage, we know that the single Mendelian factor responsible is fully recessive and located on the third chromosome. The mutant eye has functional abnormality at the retinal level—yet, as in the nonphototactic mutants, histological sections of retinal structure of the mutant eye are indistinguishable from sections of wild-type eye under the light microscope.