The analysis of mtDNA restriction fragments from Irish house mice revealed much polymorphism, both within and between populations. Many phenotypes showed geographical localization and there was a strong correlation between geographical distance and genetic divergence. Populations, which are discontinuous and limited to buildings or their vicinity, are apparently the result of short-range migration. Transport by man, with whom the species is closely associated, appears to have negligible impact. There is some evidence of the influence of topographical features on migration and consequent genetic interchange.