The placental alkaline phosphatase types have been determined in a series of 374 Japanese placentae. Nine different phenotypes were observed, and the gene frequencies for the common phenotypes are calculated Pls1 = 0.721, Pli1 = 0.225 and Plf1 = 0.043. In contrast to reports on Caucasians, the high Pli1 and the low Plf1 are characteristic of the present study. Rare alleles probably corresponding to Pls2 and Pls3 are also present and a new phenotype tentatively designated S1SJ is described.