Using operant conditioning methods, the pigeon's wavelength discrimination abilities were assessed in two experiments to generate discrimination functions. Both these functions showed three minima at 460, 530 and 595 nm. In the second wavelength discrimination experiment, extending measurements into the UV spectral region, pigeons also maintained good discrimination between wavelengths within the UV range tested. A fourth minimum was indicated at the lower end of the spectral range tested (365-385 nm). The results point to the complexity of the pigeon's chromatic system, which must be at least tetrachromatic, probably pentachromatic. eral times. A first function obtained by Hamilton and Coleman in 1933 had suggested that the pigeon's colour vision was similar to that of man. Later determinations (e.g., Blough, 1972; Riggs etal., 1972; Schneider, 1972; Wright, 1972a) have definitely corrected that erroneous early impression but they have not yet yielded sufficiently concordant functions. Moreover, none of the measurements has extended into the near-ultraviolet. This has become important since it has been demonstrated that pigeons are visually sensitive in this spectral region (see Kreithen, 1978; Wright, 1972b). We now report two experiments intended to improve this situation.