The populations of Mediterranean Pine Vole Pitymys duodecimcostatus were simultaneously monitored by removal trapping (Guedon & Pascal, 1992) in two agrosystems of Southern France, an apple orchard (for 5 consecutive years) and an asparagus field (for 4 years). Altogether, 1 668 individuals were captured on these two study sites. Furthermore, 7 598 control animals were captured along standard trap lines in similar agrosystems within the same kinds of habitat. The seasonal and yearly variations of vole density were estimated using an index of abundance. The trapped animals were sexed, their ages estimated using the eye lens weight method, and the reproductive condition of females ascertained by post-mortem examination. Despite an heavy damage on crops, the Mediterranean Pine Vole density remained rather low throughout the study period, as compared with species such as Microtus arvalis and Arvicola terrestris. It displayed seasonal fluctuations, but no tendency to pluriannual cycling. The sex-ratio remained balanced in all populations sampled. The percentage of voles over one year of age remained high (ca. 10 %) all the year long, whereas the percentage of animals belonging to the age class 50 days-1 year ranged from 65 to 85 %. The proportion of reproductively active females remained stable from November to May (35 to 40 %), reached its lower values in June and July (10 %), and increased again from August to October (up to a maximum of 70 % in September). The possible causes of the unexpected population stability of P. duodecimcostatus in our study areas, despite the strong seasonality of the mediterranean environment, are discussed. As in P. pinetorum, behavioural factors might well play a determinant role, both species living in family groups.