We determined blood group phenotypes of 13 blacks who were infected with Plasmodium vivax in Vietnam. All were Duffy blood group positive as compared to 40–50% Duffy positive in surveys of black blood donors in the United States. The probability that 13 of 13 were Duffy positive by chance alone was P < 0.001. All other blood groups occurred at the expected frequency. This study is further support for the hypothesis that the Duffy negative genotype (FyFy) is the basis for resistance of blacks to P. vivax.