The fine structure of the cone and rod outer segments of the toad was studied under the electron microscope after fixation in osmium tetroxlde and fixation in formaldehyde followed by chromation. In the OsO4-fixed specimens, the rod outer segment appears to be built of a stack of lobulated flattened sacs, each of which is made of two membranes of about 40 A separated by an innerspace of about 30 A. The distance between the rod sacs is about 50 A. The sacs in the cone outer segment are originated by the folding of a continuous membrane. The thickness of the membranes and width of the spaces between the cone sacs is the same as in rod, but the sac innerspace is slightly narrower in the cone (~-~ 20 A).After fixation in formaldehyde and chromation, two different dense lines (11 and l~) separated by spaces of less density appear. One of the lines, 11, has a thickness of 70 A and is less dense than the other, l~, which is 30 A thick.