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Hibernation is a key aspect of the physiological ecology of temperate zone reptiles and where suitable dens are present, communal hibernation and long-term den fidelity may be expected. We studied long-term communal occupancy of hibernation dens in Italy and France by aspic vipers, Vipera aspis. Long-term trends were evaluated using regression analysis of the annual numbers of V. aspis at dens as dependent variables against year as the independent variable. The regression coefficients were tested against a 0 hypothetical coefficient, indicative of population stability. The results indicated that in Italy den numbers were stable from 1987-2000. However, after this period to 2017 there was a steep decline in den occupancy as indicated by a negative regression coefficient that differed significantly from 0. These declines correlated with increasing temperatures and shortened hibernation periods from 2000 and agreed with the general decline in viper numbers at the study area. At the smaller den in France, V. aspis numbers declined significantly during the period of observation and the den was abandoned by the 5th year. This was attributed to absence of females due to mortality of one of the two females and parturition in the second female. However, in contrast to the situation in Italy the general population in the locality was apparently stable over the period of observation.