Genetic variation of human senses within the normal range probably exists but usually cannot be investigated in detail for lack of appropriate methods. The study of subtle perceptual differences in red-green colour vision is feasible since both photopigment genotypes and psychophysical phenotypes can be assessed by sophisticated techniques. Red-green colour vision in humans is mediated by two different visual pigments: red (long-wavelength sensitive) and green (middle-wavelength sensitive). The apoproteins of these highly homologous photopigments are encoded by genes on the X chromosome. Colour matches of males with normal colour vision fall into two main groups that appear to be transmitted by X-linked inheritance. This difference in colour matching is likely to reflect small variations in the absorption maxima of visual pigments, suggesting the presence of two common variants of the red and/or green visual pigments that differ in spectral positioning. We report that a common single amino-acid polymorphism (62% Ser, 38% Ala) at residue 180 of the X-linked red visual pigment explains the finding of two major groups in the distribution of colour matching among males with normal colour vision.