WITH the objective of analysing visual processes, we have attempted to generate mutants of Drosophila melanogaster with single-step lesions on the genes controlling the visual pathway1. One of these (x-7) had an abnormal electroretinogram (ERG) consisting apparently of responses from the photoreceptor layer alone. Similar ERGs could be obtained from the mutants tan1,2 and ebony2. Indeed, the mutants x-7 and tan seemed to be allelic1. We now have new mutants, and ERG characteristics suggest genetic blocks in the visual pathway of several of them.