d-Glucose entry into erythrocytes from adult grey-headed flying fox fruit bats (Pteropus poliocephalus) was rapid and showed saturation at high substrate concentrations. Kinetic parameters were estimated from the concentration dependence of initial rates of zero-trans d-glucose entry at 5.5 o C as Michaelis constant (K m ) 1.64+/-0.56 mM, and maximal velocity (V m a x ) 1162+/-152 μmol.l.cell water - 1 .min - 1 . d-Glucose entry was inhibited by cytochalasin B; mass law analysis of d-glucose-displaceable cytochalasin B binding gave values of K d 37.1+/-5.0 nM and B m a x 361.2+/-9.1 pmol/mg membrane protein. Entry of 2-deoxy-d-glucose, and 3-O-methyl-d-glucose, into P. poliocephalus red cells was rapid, entry of d-fructose was very slow. Glucose transporter polypeptides were identified on immunoblots as a band M r 47 000-54 000 and their identity confirmed by d-glucose-sensitive photolabeling of membranes with [ 3 H]-cytochalasin B. Peptide-N-glycanase F digestion of both human and bat erythrocyte membranes generated GLUT-1-derived bands M r 37 000. Trypsin digestion of human and fruit bat erythrocyte membranes generated fragmentation patterns consistent with similar GLUT-1 polypeptide structures in both species. Erythrocytes from adult Australian ghost bats (Macroderma gigas), a carnivorous microchiropteran bat, also expressed high levels of GLUT-1.