The incremental threshold of the isolated rod visual system is believed, under certain conditions, to obey Weber's law (that is, to increase in direct proportion to the intensity of the background). This relation was tested at several background wavelengths, over an intensity range for which the target was seen only by the rods. Although the slope on long-wavelength background approximates unity (that is, Weber's law on log-log coordinates), it averages less than 0.8 on short- and middle-wavelength backgrounds. This is the same value as that found for the thresholds of a typical, complete achromat--who lacks cone vision--regardless of background wavelength. These results force the conclusion that Weber's law for incremental threshold detection is achieved not by the rods alone but only by the rods acting together with the cones.