The photoreceptor neurons (R cells) of the Drosophila compound eye elaborate a precise array of neuronal connections in the brain. These projections exhibit target specificity and create topographic maps (retinotopy). We have screened histologically for mutations disrupting R cell connectivity in developing tissue. Eighty mutations were isolated from over 6000 ethylmethane sulfonate-mutagenized lines. Characterization of these mutations included genetic mosaic analysis to determine whether the gene is required in the retina or in the optic ganglia. Most mutations were found to affect connectivity indirectly by disrupting development more generally in the eye or brain. Genes were identified as candidates for playing direct roles in R cell connectivity by affecting axonal outgrowth (eddy), target recognition (limbo and nonstop), and retinotopy (limbo).