Brightness discrimination of black-white (in the range of 0.0082-0.214 W/m2; CIE chromaticity coordinates X = 0.340, Y = 0.354) and red (in the range of 0.0035-0.106 W/m2; CIE chromaticity coordinates X = 0.641, Y = 0.342) stimuli was studied during using choice conditioning paradigm in two carps. Confusion matrices for black-white and red stimuli separately were constructed from probabilities of instrumental responses (catching of a bead) to conditioned and differential stimuli in sequential six series in which one of ten intensities used in the experiment served as a conditioned stimulus. Confusion matrices were transformed into correlation matrices between the vectors which corresponded to the applied stimuli. Principal component analysis revealed two-dimensional circular structure of carp's brightness perceptual space both for achromatic and for red stimuli. This space was principally similar to that in humans. Two eigenvectors which constitute two-dimensional Euclidean space of brightness may be interpreted as bright and dark neuronal channels.