Many abiotic environmental factors elicit the production of stress-ethylene in higher plants. To elucidate the molecular mechanisms underlying the regulation of stress-ethylene production and the physiological roles played by stress-ethylene in stress responses of plants, we studied the gene expression of ACC synthase in tobacco plants that had been subjected to environmental stresses. Four new tobacco ACC synthase cDNA fragments, NT-ACS2, NT-ACS3, NT-ACS4 and NT-ACS5, were identified and sequenced. It was found that NT-ACS2 could be induced by wounding, cold temperature and, especially, sunlight. NT-ACS4 was induced at a faster kinetics by wounding. The multiple environmental stress-responsive (MESR) NT-ACS2 gene was found to contain three introns and four exons and encode a polypeptide of 484 amino acids, 54·6 kDa and pI 6·87. Computer analysis of the 3·4 kb 5′ flanking region upstream of the ACS coding region revealed the existence of a group of putative cis-acting regulatory elements potentially conferring wounding, chilling, and UV light inducibility. Phylogenetic analysis of ACC synthase genes of different plant origins indicated that the chill-inducible NT-ACS2 gene is closely related to a chilling-inducible citrus ACS gene.