Research is being actively conducted into what causes fluctuations of small mammal populations. Norrdhat and Korpimaki [(2002). Changes in population structure and reproduction during a 3-yr population cycle of votes. Oiks 96, 331-345] propose an approach which ranks the various regulating factors by order of importance. They claim that the two most influential parameters are survival rate and reproductive rate. Here, we analyze population structures and reproductive capacities of a cyclic population of water vote (Arvicola terrestris) in a highland region (Franche-Comte, France) where regular outbreaks occur. The results reveal an ageing of the population, beginning during the abundance phase and continuing through the decline phase. This alteration in the age structure is not concomitant with any reduction in the reproductive capacity of either mates or females. However, a deficit in the cohort of juveniles is observed during the decline stage. Our study yields evidence that factors affecting the rearing or survival of young individuals are operative at an early stage in the population cycle, well before the onset of the decline phase.