To study role of abscisic acid (ABA) in cold acclimation and cold/freezing tolerance in wheat, we analyzed an ABA-hypersensitive mutant of Triticum aestivum, named ABA27. ABA-hypersensitivity in ABA27 was confirmed by bioassays involving germination and seedling growth and expression analysis of ABA-responsive genes in comparison with the parental cultivar 'Chihoku-komugi' (Chihoku). ABA27 showed significantly increased freezing tolerance in seedlings without cold acclimation. ABA-treated seedlings of ABA27 accumulated more transcripts of ABA-responsive genes Cor/Lea and their putative transcription factor (TF) genes than Chihoku under both normal and low-temperature (LT) conditions. Non-ABA-regulated Cor/Lea transcripts showed higher accumulation in ABA27 also under normal temperature. These results suggest that the elevated ABA sensitivity in ABA27 contributes to the improved freezing tolerance through increased expression of the ABA-regulated LT signal pathway. Based on these and previous results obtained in an ABA-less-sensitive mutant, it is suggested that both positive and negative regulation of ABA response is involved in the basic mechanism of freezing tolerance in wheat.