Change is traced in the biosocial structure of the Japanese American population of Seattle-King County, Washington, from that of a deme to a state approaching panmixis with surrounding American populations. Data from 2,767 marriage license applications of Japanese Americans between 1930 and 1975 were used to calculate rates of racial exogamy. Rates rose from prewar levels of less than 1% to over 50% in 1975. The total Japanese American county population was subdivided into a nuclear group, tracing its origins back to the original founding immigrants, and a nonnuclear group which has migrated into the area since World War II. Until 1965, the nonnuclear group consistently led the nuclear group in exogamous marriages. In both groups, females consistently out-married more frequently than did the males. Further analysis of community exogamy, i.e., nuclear member marriage to a nonnuclear Japanese American, showed that this type of exogamy within the second generation preceded the high rates of racial exogamy characterizing the marriage patterns of the third generation. Familial analysis suggested that the maternal kin ties of nuclear population females continue to influence marriage choice of their offspring. The patterns of increasing racial exogamy for Japanese Americans are examined against their rise in socioeconomic status, [biosocial structure, exogamy, population]