An annual study of the bacterioplankton community structure was carried out at Stn L4 (50° 15'N, 04° 13'W) in the western English Channel between August 2003 and July 2004. Bacterioplankton abundance and community structure were assessed using flow cytometry and fluorescence in situ hybridization (FISH) with rRNA-targeted oligonucleotide probes, respectively. The Eubacteria domain dominated over the Archaea domain (<15%) at the highest phylogenetic level. The Sphingo-bacteria-Flavobacteria group of the Bacteroidetes phylum (SFB) numerically dominated in spring and early summer. The α-Proteobacteria dominated from late summer to winter. The SAR11 clade represented -13% of the microbial community throughout the year and accounted for up to 69% of α-Proteobacteria in late spring. Annually, γ-Proteobacteria were 2 or 3 times less abundant than the other groups and showed no obvious seasonal trend. The SAR86 cluster accounted for up to half of γ-Proteobacteria when it peaked in summer. Consequently, we found that community structure at higher taxonomic level did not change dramatically with season but lower level phylogenetic groups showed pronounced seasonal peaks.