Stomata are pores on leaf surfaces that regulate gas exchange and close in response to elevated concentrations of atmospheric CO2. We are interested in identifying components involved in this signalling pathway and have used an infrared thermal imaging camera to screen 20,000 plants from an Arabidopsis EMS M2 population to identify mutants with an abnormal response to a short-term exposure to elevated CO2. In a specially constructed Teflon covered chamber plants were subjected to a period at ambient [CO2] (360 ppm) after which a thermal image was taken and then to a period at elevated [CO2] (up to 1400 ppm) after which another thermal image was taken. Comparing leaf temperature at ambient and elevated CO2 was possible to detect 72 mutants with abnormal leaf temperature. From the 41 fertile lines (M3) we selected three lines with the stronger phenotype:two are recessive mutations, while one is a dominant. One of the recessive mutations has a leaf temperature higher then wild type at ambient CO2 concentration (360 ppm) suggesting that the stomata are more closed then in wild type plants. The other two mutants have leaf temperature cooler then wild type and remain so at elevated CO2 (1500 ppm) suggesting the inability of the stomata to close as they do in the wild type plants.