Technical Abstract: A modular and novel assay system for rapid mass screening of chemical compounds for contact irritant and spatial repellent actions against adult mosquitoes is described. The responses of Aedes aegypti to various concentrations of three topical repellents, DEET, Bayrepel, and SS220, were evaluated. At treatment concentrations ' 25 nmoles/cm2 of SS220, mosquitoes exhibited significant contact irritant (escape) and spatial repellent (movement away from the chemical source) responses. In contrast, a treatment concentration of 250 nmole/cm2 of DEET and Bayrepel resulted in responses that were not statistically significant or just moderately significant. The novel bioassay system detected contact irritancy and spatial repellency activity with reproducible results, and provided baseline data for determining minimum effective concentrations for other chemicals. The system is compact in size, easy to decontaminate, and requires only a minute quantity of chemical compound.