Chilling injury was circumvented by heat-treating mature green tomatoes (Lycopersicon esculentum, cv. Mountain Springs) at 42 degrees C for two days prior to storing them at 2 degrees C for one or two weeks, whereas fruits stored at 2 degrees C without preheating developed typical chilling injury symptoms and failed to ripen at 20 degrees C. Using mRNA differential display and screening of the cDNA libraries, we have cloned from tomato fruit a full-length HCT1 cDNA (heat induced/chilling tolerance related). The protein ( 17.6 kDa) predicted from coding region of HCT1 cDNA has high identity with class II cytosolic small HSPs. The gene corresponding to HCT1 cDNA was termed as LeHSP 17.6. Southern-blot hybridization indicates that LeHSP 17.6 belongs to a two-member gene family. Northern blot analysis indicates the heat-induced transcript of the LeHSP 17.6 remains up-regulated during subsequent exposure of the fruit to chilling temperatures for at least one week and upon transfer to ripening temperatures for one day. Fruits which were only chilled show a low level of expression of the LeHSP 17.6 transcript. We hypothesize that LeHSP 17.6 may be involved in protecting the cell from metabolic dysfunctions leading to ripening failure caused by chilling injury. This is the first report of a class II cytosolic smHSPs encoding gene in tomato.