Trichromatic colour vision depends on the presence of three types of cone photopigment. Trichromacy is the norm for all Old World monkeys, apes and humans, but in several genera of New World monkeys, colour vision is strikingly polymorphic. The difference in colour vision between these New and Old World primates results form differing arrangements of the pigment genes on the X chromosome. In Old World primates the three photopigments required for routine trichromatic colour vision are encoded by two or more X-chromosome pigment genes and an autosomal pigment gene. New World monkeys typically have only one X-chromosome pigment gene; multiple alleles allow different types of dichromatic colour vision and, in female heterozygous at this locus, variant forms of trichromatic colour vision. Here we report that multiple X-chromosome pigment genes and trichromatic colour vision are the norm for one genus of platyrrhine monkey, the howler monkey, Alouatta.