In the developing nervous system of Drosophila, cells in each proneural cluster choose between neural and epidermal cell fates. The neurogenic genes mediate the cell-cell communication process whereby one cell adopts the neural cell fate and prevents other cells in the cluster from becoming neural. In the absence of neurogenic gene function, most, if not all of the cells become neural. big brain is a neurogenic gene that encodes a protein with sequence similarity to known channel proteins. It is unique among the neurogenic genes in that previous genetic studies have not revealed any interaction between big brain and the other neurogenic genes. Furthermore, the neural hypertrophy in big brain mutant embryos is less severe than that in embryos mutant for other neurogenic genes. In this paper, we show by antibody staining that bib is expressed in tissues that give rise to neural precursors and in other tissues that are affected by loss of neurogenic gene function. By immunoelectron microscopy, we found that bib is associated with the plasma membrane and concentrated in apical adherens junctions as well as in small cytoplasmic vesicles. Using mosaic analysis in the adult, we demonstrate that big brain activity is required autonomously in epidermal precursors to prevent neural development. Finally, we demonstrate that ectopically expressed big brain acts synergistically with ectopically expressed Delta and Notch, providing the first evidence that big brain may function by augmenting the activity of the Delta-Notch pathway. These results are consistent with bib acting as a channel protein in proneural cluster cells that adopt the epidermal cell fate, and serving a necessary function in the response of these cells to the lateral inhibition signal.