An analysis of allelic variation at genetic loci controlling several esterases and hemoglobin, as demonstrated by electrophoresis, indicates that wild populations of the house mouse (Mus musculus) are characterized by fine-scale genetic subdivision, which, through the territorial behavior of family groups (tribes), is achieved even in the absence of physical or ecological barriers to migration. Heterogeneity in allele frequencies among samples from farms in the same region and from barns on the same farm was demonstrated. Spatial variation in allele frequencies within single barns, involving a clustering of like genotypes, was shown by grid-trapping, thus providing direct evidence of tribal subdivision in continuously distributed populations. For two loci, Es-3 and Hbb, an excess of heterozygotes appeared in samples from small populations, while a deficit characterized samples from large populations. The evolutionary significance of subdivision and consequent drift in house mouse populations cannot properly be evaluated at this time. Although stochastic processes may play the dominant role in determining, at a given locus, the genotypes of individuals and frequencies of alleles in small populations, geographic patterns of variation, as studied in Texas, are characterized by uniformity of allelic frequency in major physiographic or climatic regions, as would be expected if selection is determining the frequencies.