The critical durations for temporal integration at threshold were obtained for lights of various wavelengths, presented both against darkness and against achromatic or chromatic backgrounds of different luminances. The critical duration (tc) was defined by a point of intersection of the two lines with the slopes of zero and unity in a log I.t-t plot, fitted by means of an algorithm implemented by computer program. tc was short for the lights of middle wavelengths, longer for the lights at the ends of the spectrum, and became shorter as the background luminance increased. tc also depended on the background wavelength, generally showing color-opponent characteristics when the difference was taken between tc's for the chromatic background and those for the achromatic one. The results were interpreted as the manifestation of chromatically-opponent system activities in temporal integration.