The output of domestic manufacturing plants exhibited increasing relative concentration between 1939 and 1958, but in 1958 the degree of relative concentration declined slightly from the 1954 level. The results do not reflect manufacturing company or ownership distributions. The trend in relative concentration of output seems to reflect changes in average value added by manufacture for the establishment size classes. During the 1939–1954 period the number of smallest size establishments increased much more rapidly than the largest size establishments. However, the average contribution to industrial production of the smallest establishments showed a much lower rate of increase than for the largest establishments. The 1954–1958 period was characterized by similar growth patterns for all establishment size classes.