Drosophila melanogaster is an ideal model organism to study genetic regulation of innate behaviors. Male Drosophila carry out a complex courtship ritual involving a specific sequence of steps, which depend on the processing of sensory cues by the CNS. fruitless (fru) is the master regulatory gene that controls sex-specific behaviors; this gene is both necessary and sufficient for all aspects of the courtship routine. We have identified a gene, Trapped in endoderm 1 (Tre1), that is essential for normal courtship behavior in male flies. Tre1 encodes an orphan G-protein-coupled receptor that is required for both cell migration and establishment of cell polarity, but has not previously been implicated in courtship. The goal of this research is to characterize the role of Tre1 in Drosophila mating behavior. We found that male flies in which Tre1-expressing cells are feminized through expression of the female specific isoform of the splicing factor Transformer (TraF), display unusually rapid courtship initiation. Additionally, partial loss-of-function mutants in Tre1 show an aberrant courtship behavior, including rapid courtship initiation. To establish whether this phenotype resulted in increased reproductive fitness, we performed fertility assays and found that male flies with feminized Tre1-expressing cells were at a competitive reproductive advantage, while the Tre1 hypomorphs were at a competitive reproductive disadvantage, indicating a complex requirement for Tre1 in courtship success. Finally, we found that Tre1-GAL4 is expressed in a sexually dimorphic pattern in the olfactory organs and olfactory centers of the CNS: the antennae, antennal lobe, and lateral horn, suggesting a role for Tre1 in the processing and integration of sensory signals during courtship. Future experiments will involve characterization of the Tre1 signal transduction pathway, specifically, determination of the Tre1 receptor ligand and downstream targets. This will further implicate Tre1’s role in Drosophila courtship. Additionally, to determine whether feminization of Tre1-GAL4 expressing cells leads to a gain-of-function or loss-of-function effect, we will perform experiments driving the expression of a temperature-sensitive allele of shibire, and the heat-sensitive TrpA1 channel in Tre1-GAL4 neurons. We will present the results of feminization of Tre1-GAL4 expressing neurons, as well as the effects of mutations of Tre1 on courtship behavior. We will also describe how feminization and mutation of Tre1 affect the fertility of male Drosophila. Furthermore, we will show confocal images of the sexually dimorphic expression pattern of Tre1-GAL4 in the CNS.