During monthly bat surveys carried out in winters of 2008/2009 and 2009/2010 we studied clustering behavior of greater mouseeared bats (Myotis myotis) hibernating in the Międzyrzecz Fortified Front (MFF) in western Poland. Since the behavior of hibernating bats is usually affected by varying environmental conditions we measured changes in the ambient temperature (T_a and water vapor pressure (WVP) and their variability in the selected areas and analyzed the relationship between clustering behavior of hibernating bats and abiotic conditions. In both winters, the number of solitarily roosting individuals of M. myotis decreased from autumn to spring while the highest number of bats hibernating in clusters was recorded in the middle of winter. The number of clusters did not change significantly over the winter, but the number of individuals within a particular cluster increased from November (median = 5, inter-quartile range, IQR = 5-8) to March (median = 20, IQR = 14-35.5). The changes of the clusters' size were best explained by a mixed model with WVP and the variability in WVP over the 20 days prior to the bat survey as explanatory variables. As WVP and the variability in WVP decreased, the number of individuals in a cluster increased. Also, T_a affected the size of clusters. However, neither of the models supported the hypothesis of the effect of variability of T_a on clustering of M. myotis. We propose that huddling enables bats to reduce evaporative water loss during the middle and at the end of the hibernation and reduces costs of spring arousals, perhaps by synchronizing them between clustered individuals and thus allowing the use of passive re-warming.