Winning chances of fast- and slow attacking genotypes were determined in contests of wild male house mice with standard inbred conspecifics, using a variety of conditions. Genotype-environment interactions are shown to determine winning chances. Dif- ferences are connected with social experience, familiarity of terrain, exploratory behaviour and relative weight. It is argued that, as a result, the fitness of slow attack- ing males is higher than that of fast attackers in unfamiliar environmental conditions (e.g. when migrating), but lower when on familiar ground. This leads to diversifying selection, which may explain the distribution of attack latency scores found in wild house mice