Glutathione S-transferases (GST) form a large family of non-photosynthetic enzymes known to function in detoxification of xenobiotics. We have cloned and characterized a novel, low temperature regulated GST, Solanum commersonii glutathione S-transferase (Scgst1), from a cold acclimated wild potato species S. commersonii and studied the level of its transcription in freezing tolerant and sensitive Solanum genotypes. Active oxygen species (AOS) were associated with the early steps of Scgst1 regulation since a strong mRNA signal was detected in hydrogen peroxide and salicylic acid treated plants. In experimental conditions where the formation of AOS is known to accelerate, such as excessive light at low temperature, significant accumulation of the transcript was observed in S. commersonii. Under similar experimental conditions, Scgst1 transcript did not accumulate in freezing sensitive S. tuberosum eventhough a single copy of the Scgst1 sequence was present in both species. Thus, Scgst1 in the S. tuberosum genome did not exhibit the same cold-induction properties as in S. commersonii. In comparison with the parental lines, the somatic hybrid SH9A (S. commersonii (+) S. tuberosum) had an interparental level of Scgst1 accumulation as well as freezing tolerance. The abundance of Scgst1 transcript thus correlated well with the freezing tolerance of the parental lines and the somatic hybrid SH9A. Increased GST enzyme activity was observed in S. commersonii and SH9A after 2 days of cold acclimation whereas the activity declined in S. tuberosum during the same period. Further studies of potato lines (S1) that were derived by selfing the somatic hybrid revealed a more complex relationship between freezing tolerance and Scgst1 expression level. In the S1 genotypes, the regulation of Scgst1 transcription resembled more that of S. tuberosum and was not directly related to their freezing tolerance. This could be due to the interaction of the two genomes in S1 genotypes as well as chromosomal rearrangements during meiosis.