It has been shown that when Arabidopsis thaliana plants from different populations of Karelia are grown in laboratory conditions, the time of their flowering depends on vernalization, i.e. the effect of low temperatures of varying duration. This adaptive trait proved to have populational polymorphism. The Konchezero and Shuiskaya A. thaliana populations featured a high frequency of late-flowering forms (without a cold pre-treatment, plants began to bloom in 4-6 months). The Tsarevichi population turned out to be heterogeneous in terms of flowering (both late-flowering and early-flowering forms). The populations also differed in the response to vernalization. We analyzed the dynamics of the transcription level of the histone methyltransferase gene CLF (CURLY LEAF) during vernalization in A. thaliana plants from three populations in Karelia and found that during cold exposure, the transcriptional activity of CLF tended to gradually increase towards the 20th and 30th days of plant vernalization and then significantly declined towards the 40th day (p < 0.01). The ANOVA results demonstrate that the pattern of changes in CLF transcription during vernalization did not depend on the population to which the plants belonged (F = 1.1; p = 0.34; η = 0.02). Comparison of the dynamics of CLF gene expression with the transcription dynamics of the FLC gene, which controls flowering time, suggests that histone methyltransferase CLF is involved in regulating FLC gene expression during vernalization of A. thaliana plants.