SUMMARY.1In a previous paper the swamps of the Paraguayan Chaco have been shown to be an environment in which the fauna is exposed to great lack of dissolved oxygen. The adaptations by which the rich fauna of fishes in these swamps maintains its life under these conditions are discussed in this paper.2There are two accessory sources from which these fishes can obtain oxygen. These are:—(a) The air above the water.(b) The thin, well oxygenated, surface layer of the water.Eight of twenty species collected in these swamps are adapted to breathe air and the rest all make use of the second of these sources.3. The eight air‐breathing species belong to seven genera and have six independent types of aerial respiration. The adaptations of two of these (Lepidosiren and Symbranchus) are not further discussed. In the other genera the organs of aerial respiration are:—Erythrinus Air‐bladder.Hypopomus Gill‐lamellæ.Callichthys and Hoplosternum Intestine.Anc