Geckos comprise both nocturnal and diurnal genera, and between these categories there are several transitions. As all geckos depend on their visual sense for prey capture, they are promising subjects for comparison of morphological modifications of visual cells adapted to very different photic environments. Retinae of 22 species belonging to 15 genera with different activity periods are examined electron microscopically. Scotopic and photopic vision in geckos is not divided between "classical" rods and cones, respectively; both are performed by one basic visual cell type. Independent of the activity periods of the individual species, the visual cells of geckos exhibit characteristics of cones at all levels of their ultrastructure. Thus, gecko retinae have to be classified as cone retinae. Only the large size and the shape of the photoreceptor outer segments in nocturnal geckos are reminiscent of rods; the outer segments are up to 60 microm in length and up to 10 microm in diameter. The visual cells of diurnal geckos have considerably smaller outer segments with lengths ranging from 6 to 12 microm and diameters ranging from 1.3 to 2.1 microm. Nocturnal and diurnal species differ in the structure of their ellipsoids. One type of visual cell in nocturnal geckos has modified mitochondria with either rudimentary cristae or no cristae at all, and one type of visual cell in diurnal geckos possesses an oil droplet. The visual cells of Phelsuma guentheri and Rhoptropus barnardi are intermediate between those of nocturnal and diurnal species.