Surfactant occurs in cyclically inflating and deflating, gas-holding structures of vertebrates to reduce the surface tension of the inner fluid lining, thereby preventing collapse and decreasing the work of inflation. Here we determined the presence of surfactant in material lavaged from the airspace in the gas mantle of the pulmonate snail Helix aspersa. Surfactant is characterized by the presence of disaturated phospholipid (DSP), especially disaturated phosphatidylcholine (PC), lavaged from the airspace, by the presence of lamellated osmiophilic bodies (LBs) in the airspaces and epithelial tissue, and by the ability of the lavage to reduce surface tension of fluid in a surface balance. Lavage had a DSP/phospholipid (PL) ratio of 0.085, compared to 0.011 in membranes, with the major PL being PC (45.3%). Cholesterol, the primary fluidizer for pulmonary surfactant, was similar in lavage and in lipids extracted from cell homogenates (cholesterol/PL: 0.04 and 0. 03, respectively). LBs were found in the tissues and airspaces. The surface activity of the lavage material is defined as the ability to reduce surface tension under compression to values much lower than that of water. In addition, surface-active lipids will vary surface tension, increasing it upon inspiration as the surface area expands. By these criteria, the surface activity of lavaged material was poor and most similar to that shown by pulmonary lavage of fish and toads. Snail surfactant displays structures, a biochemical PL profile, and biophysical properties similar to surfactant obtained from primitive fish, teleost swim bladders, the lung of the Dipnoan Neoceratodus forsteri, and the amphibian Bufo marinus. However, the cholesterol/PL and cholesterol/DSP ratios are more similar to the amphibian B. marinus than to the fish, and this similarity may indicate a crucial physicochemical relationship for these lipids.