Abstract.Gas bubble disease in larval striped bass,Morone saxatilis(Walbaum), was characterized by overinflation of the swimbladder and the formation of intestinal bubbles. This accumulation of gas hindered normal swimming of the fish and in extreme cases resulted in floating fish. As gas accumulated in the gut, the inner mucosal lining was compressed down into a cuboidal epithelium and in more advanced cases nearly the entire digestive tract was reduced to a squamous inner epithelium surrounded by a thin serosa. A significant increase in swimbladder volume was observed at total gas pressures as low as 102.9% and mortality was increased at 105.6‐106.0%. The older larvae (30‐day‐old) were less sensitive to gas supersaturation than 10‐ to 19‐day