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Minimum surface tension and phospholipid composition of fish lung and swim-bladder wash and tissue extract were determined on an obligate water breather, Hoplias malabaricus, two species of facultative air breathers, Hoplerythrinus unitaeniatus and Erythrinus erythrinus, and two species of obligate air breathers, Arapaima gigas and Lepidosiren paradoxa. In addition, lung tissue slices were incubated with [1-14C]acetale and [1-14C]palmitate.All lung and swim-bladder wash lipid extracts showed a minimum surface tension of 20–25 dyn/cm (1 dyn = 10 μN). The principle phospholipid of all species is phosphatidylcholine; no phosphatidylglycerol was detected in any sample.The two obligate air breathers had higher rates of incorporation of isotope in tissue slices than facultative air breathers and nonair breathers. This observation correlates well with the greater vascular supply to their lungs.The phospholipid patterns of these fishes are significantly different from those of mammals. We speculate that these lipids recovered from saline wash may not be important in the maintenance of lung stability. Perhaps their function is keeping water from leaking into the lung and swim bladder.