Studies were made of the effects of growth temperatures and transition to colder temperature on chloroplast ultrastructure, chlorophyll accumulation, lipids and protein of two Pennisetum americanum cultivars and a P. americanum × P. purpureum biotype which differed in temperature sensitivity. All genotypes had structure and chlorophyll distribution consistent with NADP-malic enzyme C4 systems and lipid phase transitions at temperatures similar to those of other 'chilling-sensitive' plants. All accumulated less starch at low temperature and there was mobilization of starch, aggregation of thylakoids in mesophyll chloroplasts and swelling of loculi on transition from 24/19 to 18/13°C. Intolerance of Pennisetum to low temperature was clearly not due to accumulation of starch, nor were genotypic differences in temperature sensitivity related to starch. The cold susceptibility of cv. Ingrid Pearl, in contrast with the tolerance of the intraspecific and interspecific hybrids, was associated with inability to accumulate chlorophyll in the mesophyll of some leaves; fluctuations in chlorophyll a/b ratios within 5 days of transition to lower temperature; and inability to accumulate higher concentrations of soluble proteins in apparently normal leaves grown at 18/13°C. Genotypic differences in temperature sensitivity did not appear related to the physical properties of membranes, which did not change within 5 days of transition to 18/13°C.