Many temperate rodents show a break in reproductive activity to conserve energy during harsh winter conditions. However, even in species that use photoperiodic changes to adjust reproductive functions, winter breeding has been recorded frequently and most likely is caused by different responses to changes in photoperiod. Furthermore, these reproductive phenotypes show clear physiologic and morphologic differences. We investigated winter reproduction in natural populations of 2 European rodents, the yellow-necked mouse (Apodemus flavicollis) and the bank vole (Clethrionomys glareolus). Analyses of our data indicate that winter breeding was due to nonresponsive phenotypes in both sexes. A broad range of gonadal changes was found, particularly in males, and individual differences in testis size were linked to differences in body weight. Population genetic analyses provided no hint of assortative mating between the 2 phenotypes.