AbstractA pin‐point lens opacity can be detected at a frequency of 90% in weanling mice from inbred strain NAW‐wa‐2+− +/Sp. Defective lenses show abnormal thickening of the anterior epithelium localized over a region of persistent contact between the primary lens fiber mass and the epithelium.A morphological study of lens development indicates that the mutant strain clearly differs from normal as early as the twelfth day of embryonic life. In a lens sector associated with the naso‐ventral quadrant of the optic cup, cells undergoing the elongation characteristic of primary lens fibers are separated from the normal boundary of the anterior epithelium by a peculiar group of cells. These cells can still be detected on the following day; they do not differentiate into primary fibers. They contribute to the anterior epithelium instead, and the zone of elongation is asymmetrically displaced from the equator. Secondary fibers formed in this sector before birth do not completely displace the primary fibers from contact with the anterior epithelium. If secondary fibers formed during the neonatal period do not accomplish separation, abnormal thickening of the anterior epithelium begins around the end of the second week of postnatal life. The epithelial thickening and capsule‐like material deposited within it apparently provide the structural basis for the opacity detected upon gross