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Female Aedes aegypti mosquitoes show strong innate attraction to humans. This chemosensory behavior is critical to species survival because females require a blood-meal to reproduce. Humans, the preferred host of Ae. aegypti, produce a complex blend of odor cues along with carbon dioxide (CO2) that attracts females ready to bite. Mosquitoes detect these cues with heteromeric ligand-gated ion channels encoded by three different chemosensory receptor gene families. A common theme in other species is that olfactory neurons express a single receptor that defines their chemical specificity and that they extend axons that converge upon dedicated glomeruli in the first sensory processing center in the brain. Such an organization permits the brain to segregate olfactory information and monitor activity of individual glomeruli to interpret what smell has been encountered. We have discovered that Ae. aegypti uses an entirely different organizational principle for its olfactory system. Using genetic strains that label subpopulations of olfactory neurons, we found that many neurons co-express multiple members of at least two of the chemosensory receptor families. This unexpected co-expression is functional, as assessed by in vivo calcium imaging showing that a given glomerulus is activated by multiple ligands detected by different receptor families. This has direct functional consequences for mosquito behavior. Mutant mosquitoes that cannot sense CO2 can be behaviorally activated by a volatile amine that stimulates the CO2 glomerulus. This non-canonical olfactory system organization featuring overlapping receptor expression may explain the female mosquito9s robust and unbreakable attraction to humans.