The Notch gene is a member of one of the sets of genes that control the developmental pathways that specify cell fate decisions in Drosophila melanogaster. The goal of this research was to use a genetic approach to identify and characterize a gene that functions with Notch in a cell fate pathway. A candidate was found in a new mutation that causes phenotypes similar to Notch and that interacts phenotypically with Notch mutations. This mutation was identified as a mutation of the deltex gene. The deltex gene was mapped to the salivary chromosome band region 6AB. Mutant phenotypes in the wings, legs, eyes, ocelli, and bristles were demonstrated. The mutant phenotypes of deltex are consistent with the hypothesis that deltex functions in a cell fate pathway. A comparison of deltex mutant phenotypes with Notch mutant phenotypes showed that the phenotypes are very similar. Synergistic phenotypic interactions between mutations of deltex and mutations of Notch showed that deltex and Notch function together in the wings, legs, eyes, ocelli, and bristles. The results of these investigations lead to the conclusion that the deltex gene functions with the Notch gene in one or more developmental pathways to specify individual cell fate.