Through a systematic genetic screen, we have identified 55 mutations that affect the development of the PNS of Drosophila embryos. These mutations specify 13 novel and 5 previously characterized genes and define new phenotypes for 2 other known genes. Five classes of mutant phenotypes were identified in the screen: gain of neurons, loss of neurons, abnormal position of chordotonal neurons, aberrant neuronal trajectories, and abnormal morphology of neurons. Phenotypic analyses of mutations identified in this study revealed three novel aspects of PNS development. First, we have identified a novel gene that may be required to define glial versus neuronal cell identity. Second, our data indicate that neuronal migration plays an important role in pattern formation in the embryonic PNS. Third, we have identified mutations that cause a lack of sensory organs, but unlike mutations in proneural genes, do not affect the formation of sensory organ precursors. These genes may be required for key aspects of neuronal differentiation. Our studies suggest that approximately 70 essential genes are required for proper PNS development in Drosophila embryos.