Summary: The percentage occurrence of the four Landsteiner blood groups in two series of full-blooded Indians is as follows: First series, comprising 316 Indians of many tribes: group I 70.89 per cent, group II 27.21 per cent, group III 1.58 per cent, group IV 0.32 per cent. Second series, comprising 457 Indians of the Navajo tribe only: group I 72.65 per cent, group II 26.91 per cent, group III 0.22 per cent, group IV 0.22 per cent. Because of the unusually high percentage of group I, both the corpuscular and serum isoagglutination elements were studied and were found to conform to those of true group I bloods.