Data are presented on the breeding biology of Micromys minutus in Britain. It is shown that although litters are produced over a protracted period (May-December), 74% of the litters are born in August and September. Mean litter size is 5·40±0·16(n= 62), which is a significant reduction on the mean litter size of 6·75±0·40(n= 16) calculated from pre-1917 literature. Litter size was found to be constant throughout the breeding season, and there was no difference in mean litter size between two samples collected close to the northern and southern limits of the known British range. By comparing the mean litter size of samples 1–5, 6–10 and 11+ days old, no significant loss of individual mice within litters could be detected, although 12·3% of all litters died prior to weaning. This loss was predominantly of autumnal litters, cold and wet being important climatic factors that terminate the breeding season. Comparisons are made with data from other parts of the animal's range.