Visual pigment polymorphism similar to that found in primates is described in the photoreceptors of wild-caught guppies (Poecilia reticulata). Microspectrophotometric examination of retinal cells revealed rod visual pigments with a λmax close to 503 nm. Classes of cones with λmax around 410 and 465 nm were found, together with a population of pigments in the 529-579 nm range. It is in these long-wavelength cones that polymorphism occurs. Male guppies are highly polymorphic for body colour and it is possible that the cone polymorphism is related to the appreciation of the different yellow, orange and red carotenoid colour spots that are used in sexual display.