Electrophoretic types of red cell esterase D (Es D) have been determined in more than 1,000 blood samples from Tokyo. The allele frequency for Es D2 was found to be 0.342. Typing was also carried out in samples of the Ainu of Hokkaido and of the Ryukyuans of the Okinawa Island: the Es D2 frequencies were found to be 0.273 and 0.369, respectively. These values are considerably higher than those described in Europeans, Negroes and even in Asiatic Indians. The results indicate that Es D with a marked degree of heterogeneity is a valuable marker for genetic, anthropological and forensic application in Mongoloid groups.