The nucleotide sequence of a full-length cDNA, the deduced amino acid sequence, and the regulation of expression of a cold acclimation-specific gene, cas18, in cell suspension cultures of a freezing-tolerant cultivar of alfalfa (Medicago falcata cv Anik) have been determined. The deduced polypeptide, CAS18, is relatively small (17.6 kD), is highly hydrophilic, is rich in glycine and threonine, and contains two distinctive repeat elements. It exhibits homology with members of the LEA/RAB/dehydrin family of proteins, which accumulate in response to abscisic acid (ABA) or water stress. It is intriguing that cas18 is induced by neither ABA nor water stress. The cas18 cDNA hybridizes to three transcripts of 1.6, 1.4, and 1.0 kb, and the cDNA characterized here corresponds to the 1.0-kb transcript. The expression of this gene is about 30-fold greater in cold-acclimated cells than in nonacclimated cells. Although the accumulation of transcripts during cold acclimation is relatively slow, their disappearance during deacclimation is dramatically rapid, becoming undetectable in less than 5 h. Studies of nuclear run-on transcription show that cold acclimation enhances the transcription of this gene nearly 9-fold. The stability of cas18-detectable transcripts during deacclimation is considerably increased if transcription is inhibited with cordycepin. It therefore appears that low temperature regulates the expression of cas18 at both the transcriptional and posttranscriptional levels.