Population ecology and reproduction of the fat dormouse (Myoxus glis L.) were studied in central Germany in the years 1992 and 1993. Animals were captured in wooden live-traps twice a month, during trapping periods lasting three days each, and marked individually by tattooing. Demographic results show that, in three subpopulations, population structure differed extremely between 1992 and 1993. In 1992, more adult than subadult dormice occurred in the study area and the reverse was found in 1993. Subadults emerged from hibernation a little before or together with adult dormice. Adult males emerged significantly earlier than adult females. After a year with good reproductive success (1992), a total lack of reproduction was observed in 1993. Juvenile dormice were neither found in traps nor in nest-boxes. The assessment of gonadal states indicated that all males (adult and subadult) remained in a state of testicular regression throughout the year 1993. Also, no female was found lactating. Reproductive failure coincided with a lack of food resources in the autumn of 1993 (e.g. fruits of oaks and beeches). Body mass changes imply high energy expenditure prior to mating in adult males. It seems likely that males in years with low food availability do not invest energy in reproduction.