The effects of fish farming on bacterial density, biomass, community structure and their possible link to antibiotic resistance have been investigated in coastal sediments of the Ligurian Sea (Western Mediterranean). The top 2 cm of the sediment under a fish farm and control areas were analysed during summer 2000. Bacterial density and biomass were three folds higher (up to 3 x 10(10) cells/g and 2602.7 muC/g) (ANOVA, p < 0,0 1) in sediments beneath the fish cages compared to the control sediments. The number of Colony Forming Units (CFUs) for culturable bacteria and microbiological investigations indicated a shift in the relative importance of the Gram-negative bacteria in impacted sediments. Gram-positive bacteria increased their relevance in the control site where they represented up to 90% of total isolates. Antibiotic sensitivity tests showed a high percentage of resistant strains in both control and impacted sediments, which indicates a widespread antibiotic resistance within bacterial populations in areas surrounding fish farms. A high frequency of antibiotic resistance was observed for ampicillin (AMP) in impacted sediments (ANOVA, p < 0.05). Gram-negative bacteria displayed the highest resistance to ampicillin, and streptomycin (STR) (ANOVA, p < 0.05) and the shift in the structure of microbial assemblage was apparently related to the presence of Gram-negative resistant strains in fish-farm sediments. The incidence of multiple resistance patterns in bacterial isolates was also greater in impacted sediments and the presence of Bacillus strains producing antimicrobial compounds may be related to the high level of drug resistance. This study highlighted a major change occurring in the structure of the benthic bacterial community most probably due to fish farming and its close association with antibiotic resistance patterns. (C) 2003 Elsevier Science B.V. All rights reserved.