Summary The water permeability of lake trout corneas, in vitro, was found to be the same whether the data are based on the transfer of tritiated water from the epithelial to the endothelial bathing media or vice versa. This occurred despite the presence of an osmotic gradient which favored movement of water through the cornea to the solution bathing the endothelium. Permeability constants for normal and denuded corneas indicate that the endothelium offers some resistance to the transfer of water into or across the fish cornea but the major resistance to this movement is provided by the epithelial layer. Correlations between water permeability and his-topathological condition of diseased corneas are noted.