The excitation energy transfer from bacteriochlorophyll(BChl)-c self-aggregates to energy-accepting BChl-a in proteins (baseplates) in an individual photosynthetic light-harvesting complex (chlorosome) of a green filamentous photosynthetic bacterium Chloroflextis aurantiacus was successfully observed at cryogenic temperature. The ratio of intensity of the fluorescence peak of BChl-a to that of BChl-c self-aggregates in individual chlorosomes, which demonstrated relative efficiency of the excitation energy transfer, was heterogeneous between 0.09 and 0.72. This suggests that excitonic interaction between BChl-c self-aggregates and BChl-a in baseplates was heterogeneous among individual chlorosomes. (c) 2005 Elsevier B.V. All rights reserved.