We studied the distribution of ABO blood group frequencies of the Galo and Mishing subtribes of the Adi tribal cluster in East Siang District, Arunachal Pradesh, India, in order to investigate the intertribal and temporal allelic variation. Blood groups O and AB showed higher frequencies (28.4%, 27.4%) in the Galo, whereas group O (45%) was predominant in the Mishing. Allele r is significantly different in the Galo (44.6%) and Mishing (60.3%). The chi-square test indicated significant deviations from Hardy-Weinberg equilibrium. Adi tribes show high heterogeneity and indicate significant temporal variation in ABO genotype frequencies in the Galo, Mishing, and Padam, whereas the Panggi, a small isolated subtribe of Adi, show similar and stable frequencies.