Lenses from diurnal geckos, Phelsuma barbouri, Phelsuma madagascariensis grandis and Phelsuma serraticauda contain a prominent 37 kDa polypeptide (pi-crystallin) that is not present in lenses from the nocturnal geckos, Gekko gekko, Hemidactylus garnoti, Tarentola annularis, and Uroplatus henkeli. This protein was partially purified from P. serraticauda and was identified as glyceraldehyde 3-phosphate dehydrogenase (GAPD). The GAPDs, which constitute 14-24% of the lens soluble protein in the diurnal species, were highly active. The presence of this enzyme in the lenses of diurnal, but not nocturnal, geckos supports the hypothesis that the oxido-reductases found as enzyme-crystallins may function in providing protection against the increased oxidative stress to which diurnal species are exposed. These findings strongly support the concept that the recruitment of enzyme-crystallins is a selective response to changes in the visual environment.