- Abnormal development of the swimbladder is described in angelfish, Pterophyllum scalare (Lichtenstein 1823), a popular freshwater ornamental fish widely cultured in a range of strains and varieties.Non-inflated swimbladders in some individuals became apparent when larvae reached an age of 2 weeks. Affected fish were incapable of maintaining their position in the water body and sank to the tank bottom, resting on their flanks. Swimming bouts involved arduous tail movements in an attempt to maintain their position in the water body. Affected fish had slender trunks and partly folded backward dorsal and ventral fins. Histologically, affected fish had a solid cluster of hypertrophic gas gland cells and a hyperplastic rete mirabile instead of a gas-inflated swimbladder. Although it was not determined whether atmospheric air is obligatory for initial inflation of the swimbladder in larvae of this species, the evidence presented suggests that access to the water surface for this purpose in angelfish may not be obligatory. Dilated swimbladders were observed in 1-day-old larvae that were still attached to the hatching stick or had dropped to the tank bottom, with no contact to the water surface. Larvae that had been collected from the water surface displayed similarly dilated swimbladders.The possible cause of swimbladder non-inflation in angelfish is presently under investigation. Although the aetiology remains unknown, we found some alterations in gene expression that were associated with swimbladder non-inflation. The underlying factors have yet to be determined, although genomic alterations, environmental conditions or induced mutation, which involves both factors, are suspected as contributory factors.