<p>Three photosystem I (PSI) particles associated with different amounts of light-harvesting complex I (LHCI) were isolated from a phycocyanin 645 (PC645)-containing cryptophyte <i>Chroomonas placoidea</i> by Triton X-100 sucrose gradient centrifugation. Two active PSI-LHCI particles (Bands 4 and 5) were resolved from thylakoids stripped of cations only, which implied that LHC migration and the adjustment of energy distribution between photosystem II (PSII) and PSI facilitated by cations could also occur in cryptophytes, like in higher plants. Peptide analysis revealed that 19.8-, 10-, and 8.3-kDa peptides corresponding to β, α₁, and α₂ subunits of PC645, respectively, were present in Bands 4 and 5. Analyses of fluorescence and circular dichroism spectra and Gauss analyses revealed efficient energy couplings between PC645 and chlorophyll within both of these PSI-LHCI particles. The excitation energy absorbed by PC645 could be directly transferred to chlorophyll <i>a</i> in the PSI core or via the chlorophyll <i>a/c</i>-protein complex of LHCI. Based on these observations, we designated the two PSI-LHCI particles as phycocyanin-PSI complexes, which have not been reported previously. Band 4-2 was a sub-complex of Band 4 and was almost depleted of LHCI (the chlorophyll <i>a/c</i>-protein complex was designated as Band 4-1) and PC645. Therefore, Band 4-2 was recognized as the PSI core complex. Negative staining and transmission electron microscopy images of Band 4 gave a direct view that the PC645 component attached with chlorophyll-protein complexes in a regular structure as hollow ring shape. Furthermore, while previous studies have indicated that PC645 associates with PSII, our results reveal that, in this cryptophytic alga, PC645 exhibits structural—but not preferential—association with PSI, which provided new information about the architecture and function of the energy transfer system comprising phycobiliproteins and chlorophyll-protein complexes in cryptophytes.</p>