Erythrocyte glucose-6-phosphate dehydrogenase (G-6-PD) activity and electrophoresis were investigated among 154 Indians living in a region hyperendemic for malaria at Alto Xingu, Mato Grosso, Brazil. No enzyme-deficient individual was found, and all subjects belonged to enzyme type B. No statistical difference in G-6-PD levels was found between tribes and sexes. The average of G-6-PD activity of the Indians was significantly higher than the normal mean values found by the same technique in Caucasians, Negroes and Japanese of São Paulo, Brazil. The high rates of G-6-PD activity of the Indians are not correlated to an increased reticulocytosis by hypochromic anaemia and appear to be typical of Indian or Indian-mixed populations. Resistance to malaria in those populations should not involve erythrocyte G-6-PD deficiency. It is suggested that the apparent association between G-6-PD deficiency and resistance to malaria found in other populations could be a statistical accident determined by the racial correlation between the incidence of G-6-PD deficiency and sickle-cell or thalassaemic haemoglobins among Negro and Mediterranean populations