Ultrafast excitation relaxation dynamics and energy transfer processes in the siphonaxanthin (Siph)-containing green alga, Codium fragile, were examined using femtosecond time-resolved fluorescence spectroscopy. Fluorescence anisotropy decay and fluorescence lifetimes clearly indicated that an efficient Siph-to-chlorophyll (Chl) a energy transfer was achieved via the S, state of Siph with a time constant of 400 fs. Energy migration times among Chl molecules were also resolved. Siph in the chloroplasts exhibited a new absorption band at approximately 535 nm, and we proposed that this originated from a new excited state (S-x) between the S-2 and S-1 states based on fluorescence anisotropy decay. (C) 2004 Elsevier B.V. All rights reserved.