This thesis is an investigation of some of the effects of cold but nonfreezing temperatures on the physiological properties of mature green tomato fruit (Lycopersicon esculentum). The aim of this work was to investigate whether cold temperature-induced changes in the structure of biological membranes were responsible for the Induction of chilling injury and to determine whether mature green tomato fruit could be acclimated to cold temperatures. Microstructural studies of chilling sensitive (cs) tomatoes exposed to cold temperatures shoved that pitting was caused by the collapse of pericarp cells and that chilling induced pericarp cell shrinkage, a decrease in cellular components and possibly membrane disruption. Decreases in cell wall fluorescence due to chilling were observed as well. Ultrastructural studies on cs mature green tomato fruit indicated that the chloroplast was the first site of chill damage, and that after 14 days at 5C, membranes had degraded, organelles appeared swollen and cytoplasm appeared condensed. These effects were not evident in a chilling resistant (cr) cultivar. Spin labelling, fluorescence depolarization and differential scanning calorimetric studies on the microsomal membranes derived from the cr and cs cultivars indicated that the cr cultivar had a membrane structure lessprone to undergo lateral phase separations than the cs cultivar. This effect was observed in both the leaves and fruit tissue. Mature green tomato fruit was acclimated to cold temperatures as judged by color and texture measurements after a chilling period. Changes in the microsomal membranes during acclimation included expression of specific cold acclimation proteins, while no changes were evident in the proportion of phospholipids or fatty acids. Microvesiculation and breakage of the endoplasmic reticulum was identified as an effect of chilling temperatures; this may play a role in the regulation of protein expression. The effects of chilling on the ripening process of tomato fruit included an increase in pectinmethylesterase and peroxidase activities, leading to accelerated softening (senescence) of the fruit. Tomato fruit could be stored up to a month in the mature green state in a modified atmosphere (3% 02, 2% CO2) at 12.5C. Chemical treatments, applied as dips, were not successful in improving the storage life of the fruit. Considered together, these results provide evidence for the involvement of membrane disruption in the process of chilling injury.