The degree of genetic dissimilarity between inbred strains or substrains of mice may be estimated from available data concerning biochemical and immunological polymorphisms. Dissimilarities between substrains are bimodally distributed, suggesting that both genetic drift and contamination are responsible for substrain differences. Previously published data concerning open-field activity in a number of inbred strains are analyzed with a view to showing a linear dependence of behavioral differences on genetic dissimilarity; two independent sources of data show that about 13% of the variance may be thus explained, and that additional information explaining a further 6–11% is gained by taking into account the effects of albinism. These results are in moderately good agreement with the results of more conventional biometrical analyses.