Prochlorococcus marinus SS120 is an ecologically important and biochemically intriguing marine cyanobacterium. In addition to divinyl chlorophylls (DV-Chls) a and b it possesses a particular form of phycoerythrin (PE), but no other phycobilins and therefore no complete phycobilisomes. Here, a spectroscopic characterisation of these DV-Chls and PE is provided. Comparison of fluorescence quantum yields, excited state lifetimes and absorption characteristics indicate similar light-harvesting properties of the DV-Chls as their monovinyl counterparts. PE, which is present only in tiny amounts, was purified and considerably enriched. A phycourobilin to phycoerythrobilin ratio of 3:1 chromophores per (alphabeta) PE monomer is suggested. The in vitro fluorescence lifetime of PE is 1.74 ns. In vivo time-resolved fluorescence measurements with synchrotron radiation were used to investigate the possible role of PE in light-harvesting. The fluorescence decay time for PE is about 550 ps, indicating an unusually slow excitation energy transfer. The decay time slowed to 1 ns after addition of glycerol to cell cultures. The contribution of PE to total light-harvesting capacity was estimated to be about one (alphabeta) PE monomer per 330 DV-Chl b molecules. Thus, the capacity of PE to function primarily as a photosynthetic light-harvesting pigment in P. marinus SS120 is low.