Technical Abstract: 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 numbers of flying insects crossing a vertical plane defined by the light sheet. The system also allows for positional discrimination of each insect along the horizontal length defined by the light sheet. The system was successfully tested using a preliminary experimental protocol for determining preference or avoidance of groups of flying mosquitoes to attractants and repellents in a flight tunnel. System-verification testing showed that the OIC counted the number of mosquitoes that crossed the light sheet, and recorded the horizontal position and time at which each insect passed through the light sheet. Future iterations of the system are anticipated to provide reliable insect velocity data as well as two dimensional-positional data. The system provides a straightforward and reliable method for measuring and recording insect spatial and temporal information as they pass through an established plane.