Color vision in humans and other Old World primates depends on differences in the absorption properties of three spectral types of cone photoreceptors. Primate cones are linked by gap junctions, but it is not known to what extent the various cone types are electrically coupled through these junctions. Here we show, by using a combination of dye labeling and electrical recordings in the retina of macaque monkeys, that neighboring red and green cones are homologously and heterologously coupled by nonrectifying gap junctions. This indiscriminate coupling blurs the differences between red- and green-cone signals. The average junctional conductance is about 650 pS. Our calculations indicate that coupling between red and green cones may cause a modest decrease in human color discrimination with a comparable increase in luminance discrimination.