UV-B radiation (290-320 nm) was very effective at inducing posterior cortical cataracts in mice similar to those seen in the human senile lens. UV-A radiation, in contrast, was at most weakly cataractogenic. The posterior opacities induced by UV-B began to appear 5-6 months after daily exposure to 8 X 10-6 W cm-2 for 12 hr per day or 10-4 W cm-2 for 1 hr per day. Progression was more rapid following the more intense exposure rate. The cataracts developed without any apparent damage to the cornea. A hypothesis of how damage sustained by the epithelium results in posterior opacities is presented.