The influence exerted by filter-feeding activity on bacterial density by two sabellid species from the Mediterranean Sea (Ionian Sea, Italy), Branchiomma luctuosum Grube and Sabella spallanzanii Gmelin (Annelida: Polychaeta) was investigated. Clearance rates and retention efficiencies were estimated utilizing the species Vibrio alginolyticus selected on account of previous field studies and its importance in fish culture pathogenicity. The Cmax was 43.2+/-2.63 L h(-1) g(-1) DW for B. luctuosum and 12.4+/-2.22 L h(-1) g(-1) DW for S. spallanzanii. The Retention efficiency was 98% corresponding to a removed bacterial biomass of 44.8+7.88 microgC L(-1) g(-1) DW for B. luctuosum and 70% corresponding to a bacterial biomass of 23.8+2.95 microgC L(-1) g(-1) DW for S. spallanzanii. Maximum retention was recorded after 20 min for the first species and after 30 min for the second one. Present laboratory experiments represent a contribution to the knowledge of the filtration activity of the two polychaetes, characterizing the filtration process on bacterioplankton. Both species resulted extremely efficient in removing V. alginolyticus from seawater in experimental tanks, thus confirming the previous data from the field studies and suggesting their employment as biofilters of microbially contaminated waters in intensive aquaculture.