Using specific deoxyoligonucleotide probes we have discovered seasonally strong (up to ∼ 100%) dominance of bacteria hybridizing to a single probe, in near shore waters off Scripps pier (32°53′N; 117°15′W). The probes were designed from partially sequenced 16S rRNA (V3 domain) of isolated marine bacteria. The results indicate that this approach may be used for studies of bacterial populations in the marine environment. We have shown that a number of genotypes that at times are dominant in the natural assemblages are culturable (and not, ‘viable-but-unculturable’). Additionally, our data suggests that the discrepancy between viable counts and direct counts in sea water samples can be explained by low plating efficiency.