The nucleotide sequences of the genes coding for the subunits of the Photosystem I (PS I) core, PsaA and PsaB were determined for the marine prokaryotic oxyphototrophs Prochlorococcus sp. MED4 (CCMP1378), P. marinus SS120 (CCMP1375) and Synechococcus sp. WH7803. Divergence of these sequences from those of both freshwater cyanobacteria and higher plants was remarkably high, given the conserved nature of PsaA and PsaB proteins. In particular, the PsaA of marine prokaryotes showed several specific insertions and deletions with regard to known PsaA sequences. Even in between the two Prochlorococcus strains, which correspond to two genetically different ecotypes with shifted growth irradiance optima, the sequence identity was only 80.2% for PsaA and 88.9% for PsaB. Possible causes and implications of the fast evolution rates of these two PS I core subunits are discussed.