Changes in fluorescence emission spectra (293 and 77 K) of phycobilisomes from Nostoc flagelUforme during dissociation in phosphate buffer of various molarity were studied. There were three bands in 77 K fluorescence emission spectra of partly dissociated phycobilisomes: F648, F666 and F686, characteristic of C-phycocyanin (C-PC), allophycocyanin (APC) and allophycocyanin-B (APC-B), respectively. In severely dissociated phycobilisomes, the F666 band was veiy weak, which indicated that linkage between C-PC and APC was severely dissociated, most of the energy trapped by C-PC could not be transferred to APC, but some of it could be transferred directly to APC-B. The phycobiliproteins in these phycobilisomes constitute an energy transfer chain: PEC^C;PC-»APC-»AP^-B. A new model of phycobilisome structure is suggested, in which some C-PC contact directly with APC and others contact with APC-B.