The presence of specimens of Micropogonias funieri suffering from cataracts was observed in the area occupied by the population I (Vazzoler, 1971). The frequency and distribution of these specimens were analysed and the electrophoretic patterns of soluble eye-lens proteins were determined. This cataract is not of parasitic origin and was rare in specimens from the coastal area. In the lagunar region it occurs mainly in specimens of the River Baguagu, from April to November. Only young fish (200 to 250 mm) in imature or in maturation stages were affected. Two main pathways of evolution in the process seems to occur with a "radial" and a "difuse" type. The analysis of electrophoretic patterns showed 5 steps of evolution with remarkable alterations on the concentration of total soluble proteins as well as on the relative concentration of 8 proteic fractions grouped in 4 agregates in electropherograms, There is an increase in the relative concentration of the agregate I (more cathodic) and II, and a decrease of the III and IV (anodic) with the disappearance of the last two fractions of the latter in the final stages of the process. No alterations were determined in the relative concentrations of these agregates when comparing eye-lens of normal specimens with normal eye-lens of affected fish suggesting that the occurence of cataract in one eye does not cause any alteration in the normal eye. The results suggest that cataract may be the cause of natural mortality in the area, and its incidence may be due to a physiological or metabolic factor induced by environmental conditions.