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Age changes in the distribution of triceps and subscapular skinfold thicknesses in 1,077 upper class school age children of Guatemalan and European ancestry have been analyzed. No significant differences were found between these two groups although significant genetic differences in blood group frequencies have already been demonstrated. This indicates the effects of a common environment upon the developmental patterns of ethnically-different samples. The medians of Guatemalan and European males were significantly closer than those of the females, suggesting greater responsiveness of males to this common environment. Compared with children living in the USA and in London, the children of the present study displayed consistently smaller triceps skinfolds and larger subscapular. In addition, the median triceps thicknesses of the boys did not display the usual decline during the adolescent years until after age 15; which is considerably later than found among the USA and London samples.