(1) The relationships between habitat change and house mouse populations were studied by monthly live trapping in a corn-wheat-hay rotation on a small Maryland farm. (2) Population density reached 53.0/ha in a wheat/hay field in October and 25.4/ha in corn in September. Populations increased by immigration as wheat or corn grew and ripened and decreased by emigration as hay became tall and dense. (3) Survival rates were high in winter in the relatively stable habitat of the wheat/hay field; they were Iow throughout the summer in both fields, and were reduced by corn harvest, less so by wheat harvest. If they were related to population density or increase, or to breeding condition, the relationships were obscured by the overriding influence of habitat change. (4) In the spring, when the population in the hay field 'crashed,' essentially the entire population moved from long-established ranges in the hay field to the field of ripening wheat, where new ranges were established. In the new field, fewer than 30% of the old associations between individuals persisted. (5) Individual mice maintained home ranges (88.1 + 6.1 m in length) in the same general area during their residence in a field. Ranges shifted from month to month, perhaps in response to changes in populations and habitat; exploratory travels and other movements also modified home range behaviour. (6) Minimum life expectancy (residence time) was greater from November (4-5 months) than from June/July (1-2 months). Maximum individual age was 17 months. (7) The demographic pattern fell at the r extreme of the r-K continuum. Mice bred from May to October matured and produced litters rapidly, produced several litters in a season, and had a high turnover rate. (8) It was concluded that migration was a primary mechanism of population regulation in the cropfield mosaic and that it was driven by habitat change, a system in contrast to those described for house mice in confined conditions.