We examined growth differences in external and skull morphology between spring and autumn cohorts of the large Japanese field mouse (Apodemus speciosus). We analyzed 271 specimens from the Kyoto population, which had 2 discrete breeding periods, spring and autumn. Growth differences between the 2 cohorts were found in body weight and head and body length. The spring cohort grew rapidly in the summer following birth, whereas the autumn cohort showed growth inhibition during the autumn and winter soon after birth and rapid growth the following spring and summer. The centroid sizes (CSs) of the cranium and mandible showed similar growth patterns, but no significant postweaning growth was observed in ear length or hind-foot length. Cranium and mandible shape showed weak growth changes related to increases in CS. Decreases in mean body size during the winter were found only in males from the spring cohort. No significant difference was found between adult samples from the spring and autumn cohorts in either external or skull morphology. We suggest that insufficient food resources is the major factor causing the growth inhibition in the autumn cohort and postulate that winter survivability, which is related to reproductive competition, explains the body-size reduction in winter observed in males from the spring cohort.