The excitation energy transfer between carotenoid and chlorophyll (Chl) in the cytochrome b6 f complex from Bryopsis corticulans (B. corticulans), in which the carotenoid is 9-cis-a-carotene, was investigated by means of fluorescence excitation and sub-microsecond time-resolved absorption spectroscopies. The presence of efficient singlet excitation transfer from a-carotene to Chl a was found with an overall efficiency as high as _24%, meanwhile the Chl a-to-a-carotene triplet excitation transfer was also evidenced. Circular dichroism spectroscopy showed that a-carotene molecule existed in an asymmetric environment and Chl a molecule had a certain orientation in this complex.