D-Glucose entry into erythrocytes from adult dolphins (Tursiops truncatus) was rapid, showed saturation at high substrate concentrations, and demonstrated a marked stimulation by intracellular D-glucose. Kinetic parameters were estimated from the concentration dependence of initial rates of tracer entry at 6°C: for zero-trans entry, Michaelis constant (K_m) was 0.78 ± 0.10 mM and maximal velocity (V_max) was 300 ± 9 µmol · l cell water¹ · min¹; for equilibrium exchange entry, K_m was 17.5 ± 0.6 mM and V_max was 8,675 ± 96 µmol · l cell water¹ · min¹. Glucose entry was inhibited by cytochalasin B, and mass law analysis of reversible, D-glucose-displaceable, cytochalasin B binding gave values of 0.37 ± 0.03 nmol/mg membrane protein for maximal binding and 0.48 ± 0.10 µM for the dissociation constant. Dolphin glucose transporter polypeptides were identified on sodium-dodecyl sulfate-polyacrylamide gel electrophoresis immunoblots [using antibodies that recognized human glucose transporter isoform (GLUT-1)] as two molecular species, apparent relative molecular weights of 53,000 and 47,000. Identity of these polypeptides was confirmed by D-glucose-sensitive photolabeling of membranes with [³H]cytochalasin B. Digestion of both dolphin and human red blood cell membranes with glycopeptidase F led to the generation of a sharp band of relative molecular weight 46,000 derived from GLUT-1. Trypsin treatment of human and dolphin erythrocyte membranes generated fragmentation patterns consistent with similar polypeptide structures for GLUT-1 in human and dolphin red blood cells.