The swim bladder is a common target of infectious disease conditions. Primary bacterial and fungal infections due to direct invasion of the pneumatic duct occur in physostomes. A wide variety of Gram-negative and Gram-positive bacteria have been implicated in infections of the swim bladder including acid-fast Mycobacterium spp. All species of fish are potentially susceptible to infection of the swim bladder. Radiography may reveal a distended swim bladder with variable radiodensity/decreased radiolucency and potentially a detectable fluid line with horizontal beam projections. Gross lesions are confined to the swim bladder with evidence of local and visceral spread in advanced cases. Paecilomycosis is an ascomycete fungal infection of the swim bladder, which causes sporadic low mortality in farmed Atlantic salmon in Scotland and Norway. Nematodes of the family Anguillicolidae are parasites of the swim bladder of eels. Spirurid nematodes of the genus Cystidicola are parasites of the swim bladder of physosomous fishes.