As food fish production continues to increase, the frequency of disease will only continue to rise within the aquaculture industry. Add to this an increase in the regulation of treatments and resistance to available antibiotics means that alternative methods of disease protection will be required. Fish-derived antimicrobial peptides are an important part of the innate immune system because they often demonstrate potent antimicrobial properties. Piscidins are a class of antimicrobial peptides first described in hybrid striped bass (Morone chrysops
x Morone saxatilis ) but have also been identified in many other fish species. Previous work has shown broad antimicrobial activity of piscidins against Gram-positive and Gram-negative bacterial species. This study sought to determine the extent to which class I and class II piscidins inhibit biofilm formation of different Gram-negative bacteria. In general, the class I and II piscidins demonstrate potent activity against Escherichia coli and Flavobacterium columnare biofilms. The class II piscidins showed more activity against E scherichia coli and Flavobacterium columnare isolates than did the class I piscidins. The piscidins in general were much less effective against inhibiting Aeromonas hydrophila and Aeromonas veronii biofilm growth. Only the class I piscidins showed significant growth inhibition among the Aeromonas spp. examined. The potential use of antimicrobial peptides for disease prevention in food fish production will be discussed.