1. 1. Light minus dark difference spectra of leaves and chloroplasts at liquidnitrogen temperatures were recorded in a Cary spectrophotometer. The spectra showed minima at 547 nm and 557 nm, but no band was observed at 552 nm, the position of the α-band of cytochrome f at low temperatures. Similar spectra were obtained with spinach and normal peas and with a pea mutant deficient in chlorophyll. It is concluded that cytochrome b-559, but not cytochrome f is photooxidized at liquid-nitrogen temperatures. Light of wavelength 650 nm was much more effective than 710-nm light, indicating that the oxidation of cytochrome b-559 at low temperatures is driven by light absorbed by Photosystem II. 2. 2. The redox potential of the cytochrome which is photooxidized at low temperature was determined as 0.35 V at pH 7.2. 3. 3. Chloroplasts were illuminated at room temperature, frozen rapidly to the temperature of liquid nitrogen, and light minus dark difference spectra were recorded. The spectrum of untreated chloroplasts showed only one small band at 547 nm. Chloroplasts treated with 3(3,4-dichlorophenyl)-1,1-dimethylurea gave sharp bands at 548 nm and 552 nm due to cytochrome f, but no band at 557 nm. 3(3,4-Dichlorophenyl)-1,1-dimethylurea inhibited the low temperature photooxidation of cytochrome b-559 provided the chloroplasts were illuminated at room temperature before freezing. Cytochrome b-559 was photooxidized to some extent at room temperature if the chloroplasts were treated with carbonyl cyanide m-chlorophenylhydrazone. 4. 4. The results are difficult to reconcile with an electron transport scheme in which cytochrome b-559 is located on the main coupled pathway between the photoacts of Photosystems I and II. We suggest that cytochrome b-559 is on a side pathway connected to the reaction centre of Photosystem II.