Anthropometric measurements, pulmonary function, and maximal oxygen uptake determinations were obtained on 45 healthy males. The subjects were assigned to three groups of 15 each, according to racial origin and generation in a particular environment. These groups were Issei (native Japanese), X age = 24.4 years; Sansei (third-generation Japanese-Americans), X age = 19.9 years; and Caucasian (white Americans), X age = 26.7 years. The stature of Issei and Sansei was smaller than that of Caucasians. There were no differences in calculated percentage of body weight due to fat or estimated caloric intakes. Sansei had larger chest and wrist widths and thigh circumferences than those of the Issei, and smaller than or equal to those of the Caucasians. These structural differences were linked with pulmonary function and maximal oxygen uptake differences between the three groups. Comparison of the obtained data with previous data indicated a maximal average size of the Japanese to be 169 cm, which is genetically controlled. Environmental factors appeared to affect both structural differences and pulmonary dimension differences. When structural differences were considered, the Caucasians were found to be more physically fit than the Sansei, who in turn were fitter than the Issei. These differences can be explained by activity patterns