Major concerns surround the use of antimicrobial agents in farm-raised fish, including the potential impacts these uses may have on the development of antimicrobial-resistant pathogens in fish and the aquatic environment. Currently, some antimicrobial agents commonly used in aquaculture are only partially effective against select fish pathogens due to the emergence of resistant bacteria. Although reports of ineffectiveness in aquaculture due to resistant pathogens are scarce in the literature, some have reported mass mortalities in larvae caused by resistant to trimethoprim-sulfamethoxazole, chloramphenicol, erythromycin, and streptomycin. Genetic determinants of antimicrobial resistance have been described in aquaculture environments and are commonly found on mobile genetic elements which are recognized as the primary source of antimicrobial resistance for important fish pathogens. Indeed, resistance genes have been found on transferable plasmids and integrons in pathogenic bacterial species in the genera , , , , and . Class 1 integrons and IncA/C plasmids have been widely identified in important fish pathogens ( spp., spp., spp., spp., and spp.) and are thought to play a major role in the transmission of antimicrobial resistance determinants in the aquatic environment. The identification of plasmids in terrestrial pathogens ( serotypes, , and others) which have considerable homology to plasmid backbone DNA from aquatic pathogens suggests that the plasmid profiles of fish pathogens are extremely plastic and mobile and constitute a considerable reservoir for antimicrobial resistance genes for pathogens in diverse environments.