The light-harvesting protein C-phycocyanin (PC) in the trimeric aggregation state, isolated from the cyanobacterium Synechococcus sp. PCC 7002, is studied by absorption spectroscopy and by time-resolved anisotropic fluorescence spectroscopy with 1 ps time resolution. PC trimers isolated from the wild-type strain and a mutant strain, cpcB/C155S, in which the P155 chromophore is missing, are compared. The absorption spectra of the trimeric PCs, when compared with previously published spectra of the monomeric PCs [Debreczeny et al. J. Phys. Chem. 1993, 97, 9852-9862], lead us to conclude that the absorption spectrum of the beta(155) chromophore is similar when PC is in the monomeric and trimeric states. This means that the red shift of the absorption spectrum that occurs when PC aggregates from monomers to trimers is due to changes in the spectra of the alpha(84) and/or beta(84) chromophores. First-order exciton coupling between chromophores cannot alone be the cause of the red shift.