1. The plasma membrane of the dogfish erythrocyte is characterized. 2. Surface membranes were isolated from dogfish red cells. 3. Cells suspended in a hypotonic zinc chloride buffer (0.5 mM ZnCl2, 5 mM Tris-HCl, pH 8.0) tended to enucleate spontaneously. 4. By gentle homogenization relatively high yields of red cell "ghosts" were formed; these were purified using discontinuous gradients of sucrose and of glycerol solutions. Analyses of protein, carbohydrates and lipids were carried out. 5. There did not appear to be marked overall differences in the composition of the surface membrane of the nucleated dogfish erythrocyte compared to those of other species.