The hypothesis that market growth leads to reduced sellers' concentration is presented and briefly elaborated. Then, after an evaluation of the data, they are used to make a direct and an indirect test of the hypothesis. In the direct test, a negative relationship is found between concentration change and market growth, both for aggregate changes and for those of individual markets. In the indirect test concentration change is found to be related to the increasing importance of multi-product plants in a fashion that implies that the increased diversification of companies, rather than that of their plants alone, has brought lower levels of product concentration. Both tests lend positive though not conclusive support to the hypothesis.