Mitochondrial lipids from Lycopersicon hirsutum undergo a broad thermal transition beginning well below 0 degrees C and ending at approximately 25 degrees C. Differential thermal analysis of mitochondrial lipids isolated from ecotypes of L. hirsutum that differ in chilling sensitivity indicates that these lipid preparations have physically similar properties. This was confirmed by electron-spin-resonance experiments, although this technique failed to detect the broad transition detected by differential thermal analysis. No quantitative differences were observed between the percentages of individual lipid classes (based on polar head group) or between the fatty acid compositions of mitochondrial lipids from the two ecotypes investigated. These results suggest that the observed differences between the responses of these ecotypes to prolonged exposure to 5 degrees C may not be related to differences between the physical properties of their mitochondrial lipids.