Old World primates have trichromatic vision because they have three types of cone photoreceptor, each of which is maximally sensitive to short, middle or long wavelengths of light1. Although a proportion of human males (about 8% of caucasians, for example) have X-chromosome-linked colour-vision abnormalities2, no non-human Old World primates have been found to be colour-vision defective3,4. We have tested 3,153 macaque monkeys but found only three dichromats, a frequency that is much lower than in humans.