A man with a unilateral colour vision defect is reported. The defect of his right eye can best be defined as lying between deuteranopia and extreme deuteranomaly. The left eye was unusual in that it gave a normal Rayleigh equation on the anomaloscope but abnormal readings, classical for red‐green defectives, with the Ishihara test plates. Two cousins and an uncle had bilateral colour vision defects closely resembling the defect from the right eye of the proband. Various explanations were sought for the findings, but ocular pathology, abnormal sex chromosome numbers, and mosaicism with Lyonization as well as somatic back mutation all seem to be unsatisfactory as such.Three other published cases of a unilateral colour vision defect, two in males and one in a female, are discussed, as is the presence of bilateral colour vision defects in one each of three presumably monozygotic female twin pair