Population dynamics of the coypu, Myocastor coypus, were studied in central-West France. This species has recently colonized temperate regions despite physiological sensitivity to cold weather. Live-trapping over three years demonstrated that increasing population densities from summer into autumn were associated with a faster population turn-over, homogeneous dispersion, and reduced rates of weight gain. Densities fell by 71% over a cold winter in which canals froze for 20 consecutive days, and by 69% over a mild winter with a flood of 5 weeks duration. Mortality was high in the coldest weather, and 45% of those surviving had frost lesions. A scarcity of males after the cold winter (up to 1♂: 3.3♀) was matched in a nearby population at artificially low density, where dispersing males predominated after the flood. The effect of the cold winter on reproductive output was to synchronize littering in summer, at a time of rich food supply. It was concluded that a flexible response to winter conditions enabled coypus to minimize the effects of extreme events and maximize their colonizing potential.