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Several times throughout their radiation fish have evolved either lungs or swim bladders as gas‐holding structures. Lungs and swim bladders have different ontogenetic origins and can be used either for buoyancy or as an accessory respiratory organ. Therefore, the presence of air‐filled bladders or lungs in different groups of fishes is an example of convergent evolution. We propose that air breathing could not occur without the presence of a surfactant system and suggest that this system may have originated in epithelial cells lining the pharynx. Here we present new data on the surfactant system in swim bladders of three teleost fish (the air‐breathing pirarucu Arapaima gigas and tarpon Megalops cyprinoides and the non‐air‐breathing New Zealand snapper Pagrus auratus). We determined the presence of surfactant using biochemical, biophysical, and morphological analyses and determined homology using …