Some population characteristics of the mouse, Mus musculus, collected by fumigation and trapping from suburban and pastoral areas and grain stores near Christchurch from 6 March to 18 July 1970 are described. Significantly more females (83) than males (59) were caught (P < 0.05); female predominance was greater in samples of heavier (> 16.5 g) and older (> 20 weeks) mice. Both sexes became fecund at about 8 weeks old and 10.6 - 12.5 g body weight. An index of testis size (length and width) was correlated with fecundity in both laboratory and wild male mice, although values differed. Corpora lutea in the ovaries of females indicated fecundity. A significantly greater (P < 0.02) proportion of females heavier than 18.5 g were pregnant (71%) than was so in lighter females (21%). There was no increase in average proportion pregnant with increasing age. On average, 36% of females were fecund during the collection period and each produced 3.34 litters (total 19.2 embryos) in that time. Extrapolating from the sex ratio in 31 nestling mice (19 females; 12 males) 11.8 of the embryos would become daughters. Apart from the above, some of the main conclusions of this study are that the visibility of tubules in the cauda epididymis is the best indicator of male fecundity; the presence of corpora lutea in the ovaries indicates female fecundity; the breeding of wild mice in Christchurch may continue into the winter, the physical condition of females, as indicated by their weight, is more important than age in affecting fertility.