An instrumental differentiation method was used to study the discrimination of the intensities of black/white (over the range 0.0082-0.214 W/m2; coordinates in the CIE-31 system: X = 0.340, Y = 0.354) and red (over the range 0.0035-0.106 W/m2; coordinates in the CIE-31 system: X = 0.641, Y = 0.342) stimuli in two carp fish (cyprinus carpio L.). Confusion matrices were constructed using the probabilities of instrumental responses (bead seizing) on selection from a pair of stimuli (a conditioned stimulus and one of nine differentiating stimuli) in six series using a conditioned stimulus of defined brightness. Matrices of correlations between vectors (the stimuli used for the confusion matrices) were subjected to factor analysis to identify their intrinsic vectors. The perceptual brightness spaces for both black/white (two fish) and red (one fish) stimuli were found to be spherical in structure, resembling the major features of the human brightness space. The coordinate axes of this space were interpreted as representing excitation of two channels encoding intensity as brightness and darkness.