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The force of internal migration toward national genetic homo' geneity is seen in a cohort of 8850 British women who were ABO and Rh (+ or —) typed and located by residence in one of 40 county or county- group areas in England, Scotland and Wales in 1958 and again in 1974. The dines of phenotype frequencies of types O, A, B + AB and Rh— were plotted by polynomial regression on easting (longitude) and northing (lati­tude) and show a persistent heterogeneity but with statistically significant inter-regional shifts that account for the slightly flatter clinal curves for some of the geographic distributions after 16 years.