In Poland, two subspecies (morphs) of weasel Mustela nivalis exist, which differ in winter pelage colour, white in M. n. nivalis and brown in M. n. vulgaris. Weasels are vulnerable to attacks from other predators, thus cryptic coat colour may affect probability of detection, and in consequence, their mortality. We hypothesize that natural selection should favour one of phenotypes, according to prevailing weather conditions during winter. Analysis of trapping data from the Białowieża Forest, where both subspecies occur sympatrically, revealed that the proportion of M. n. vulgaris increased along with decreasing number of days with snow cover and increasing mean ambient temperature. Moreover, recently we observed increasing variation in timing of moult in M. n. nivalis, whereas in the past it was fixed and most probably regulated entirely by photoperiod. To demonstrate that white or brown winter pelage may change predation rates, we performed a field experiment using weasel models (white and brown) exposed against different background colours. This experiment revealed that contrasting models of weasels were attacked significantly more often by predators than those matching the background colour. Results of our study provide another example that in mammals variation range of phenotypic traits is big enough to enable rapid adaptations to changing climatic conditions.