Abstract Hematophagous insect vectors of disease locate their human targets in large part through chemosensory- driven behaviors. Development of new insect control measures will require an improved understanding of the molecular and cellular basis underlying their olfactory capabilities. Here, we propose to investigate the mechanisms underlying the ability of insects to detect ammonia, an odor released by humans and attractive to both insect vectors of human disease and non-hematophagous insects. We utilize Drosophila as a genetic model organism due to its wealth of genetic tools that allow us to dissect the fine workings of the system. Using our newly developed genetic tools, we have identified a previously unknown fourth neuron in strongly ammonia sensitive ac1 sensilla. The scientific premise of this proposal is that ammonia responses are mediated by these newly identified Rh50+ neurons and do not depend on the previously implicated IR92a odor receptors or IR92a+ neurons. In Aim 1, we propose to characterize the odor response profile of Rh50+ neurons and determine which neuron mediates electrophysiological and behavioral responses to ammonia. In Aim 2, we will examine whether the ammonia sensitivity of Rh50+ neurons is mediated by a novel type of odor receptor unrelated to the two previously identified odor receptor families in insects. Given the highly conserved nature of this putative receptor and its expression in the antenna of all insect species we have examined, we expect that the proposed findings regarding such ammonia-sensitive neurons and receptors in Drosophila will translate to other insect species, including vectors of human disease.