Marine cyanobacteria of the genus Prochlorococcus belong to one of two ecotypes that are specifically adapted to either low light (LL) or high light (HL) conditions. Previous analyses of the differences in pigmentation and gene complement revealed that LL-adapted ecotypes carry a gene cluster to produce a functional phycoerythrin, whereas in the fully sequenced genome of the HL-adapted strain MED4, only a single and free-standing cpeB gene occurs. This gene encodes a derived form of beta-phycoerythrin, the function of which has remained enigmatic so far. Here, an analysis of HL-adapted Prochlorococcus strains from different ocean provinces revealed the presence of a cpeB gene highly similar to that of MED4. To investigate whether the presence of particular phycoerythrin genes is a common characteristic of the LL- and HL-adapted ecotypes, primer sets targeting specific motifs in LL-cpeB and HL-cpeB were designed for polymerase chain reaction (PCR) analysis of Red Sea phytoplankton. A major PCR product for Prochlorococcus HL-cpeB was obtained from samples taken at 5-70 m depth and for LL-cpeB from 70-125 m. The high sensitivity of this approach allowed the detection of HL-cpeB down to 100 m and LL-cpeB as deep as 175 m. DNA sequence and phylogenetic analysis of 70 individual clones for HL-cpeB and of 68 clones for LL-cpeB revealed a monophyletic origin for the HL and LL sequences respectively. This study shows that cpeB sequences are suitable as very sensitive molecular markers for the study of natural populations of Prochlorococcus. The low sequence divergence of HL-cpeB among Prochlorococcus strains, which have been isolated from the Mediterranean Sea, the Arabian Sea and the Southern Pacific Ocean as well as in populations from the Red Sea, suggests the HL-cpeB gene to be conserved and its product to be functional in Prochlorococcus.