An optical insect counter (OIC) was designed and tested. The new system integrated a line-scan camera and a vertical light sheet along with data collection and image-processing software to count flying insects crossing a vertical plane defined by the light sheet. The system also discriminates each insect by its position along the horizontal length defined by the light sheet. The system was successfully tested with a preliminary experimental protocol for determining whether groups of flying mosquitoes preferred or avoided attractants and repellents in a flight tunnel. The OIC counted the number of mosquitoes that crossed the light sheet and recorded the horizontal position and time each insect passed through the light sheet. The system provides a straightforward and reliable method for measuring and recording spatial and temporal information for insects that pass through an established plane.