Mutations at the Blackpatch locus cause death and degeneration in the developing visual system of Drosophila melanogaster. Some Bpt alleles cause recessive lethality in the larval stage during development. To understand the role of Bpt in visual system development, the mutant phenotype was characterized for its effects on structure of the developing eye and the electrical activity of adult mutant eyes. The first hypothesis was that Bpt gene function may be required for the visual transduction pathway to receive and process light signals correctly. This is an electrical/biochemical process that can be visualised as an electroretinogram (ERG) and mutations in the pathway lead to abnormal ERGs. Bpt mutations may affect this pathway causing abnormal electroretinograms (ERGs). Analyses of the electroretinograms of mutant flies showed that mutant flies receive and process light correctly. Bpt does not function in the visual transduction pathway. The second hypothesis was that Bpt functions in a neurotrophic pathway. Genes in this pathway interact with signal transduction pathways necessary for cell-cell signaling and exhibit temporal and spatial specificity. Mutations in this pathway characteristically cause cell death in an apoptotic manner. In Bpt mutants, cell death in the developing eye occurs in a spatial and temporally specific manner suggesting a similar specificity for the action of the Bpt gene. The genetic focus of the mutation is in the lamina which is seen first around 42 hours post pupariation (pp) followed by death in the retina around 58 hours pp. Previous analyses has shown that Bpt interacts with Notch , an important cell signaling molecule that communicates by sending lateral inhibitory signals to its neighbors. The Bpt mutation affects the eye at a time of axonal innervation of the lamina. In mammalian systems, this phase involves the action of neurotrophic genes. Several lines of evidence suggest that Bpt may indeed be working in a neurotrophic pathway to maintain the laminar cells as they complete development;In the developing fly, Bpt mutations cause lethality in the homozygous condition. The lethal phase for Bpt is between 36 to 48 hours during the larval II instar phase of development. This suggests a time when the Bpt gene product is needed. The lethal phase is at a time of proliferation and development in the nervous system. Our results show that the morphology of the nervous system is normal at the end of embryogenesis. Bpt is necessary at a very specific time in development and any abnormalities due to Bpt take place during this time window. It is possible that the systemic death of the larva is due to multiple primary and secondary effects of the Bpt mutation. We suggest that Bpt has a vital function and this may be involved in the proliferation of the central brain neuroblasts during development;Blackpatch plays an important role in the development of Drosophila. In the adult visual system development it may be acting within a neurotrophic pathway that is necessary for maturation and maintenance when the retinal axons are innervating the lamina. In the larva, Bpt may again be functioning as part of a neurotrophic pathway in central nervous system development.