The aim of this research was to induce, at will in the laboratory, aestivation of the Dipnoan Protopterus annectens, in order to compare the structure of organs in lungfish adapted to aquatic or aestivating conditions. The animals were placed in a glass tank containing warm water, and the bottom of the tank was filled with clay and sand. To start aestivation the water was allowed to slowly evaporate; as soon as the fish was in a dry environment, it began to excavate a hole in the mud and to burrow into it. Scanning electron microscopy and histological techniques compared the morphology of skin, gills, and lungs in aestivating and free-swimming animals. In the aestivating animals, the secondary lamellae of the gills became thick and were covered by mucus that pasted the lamellae together. The epidermis of the skin was thin and composed of layers of flattened cells. In contrast, in free-swimming animals, the secondary lamellae of the gills were widely separated and the epidermis of the skin was thick and contained numerous mucus-laden cells. The lungs, thin bloodless threads in the aquatic conditions, were, in the air-breathing animals, rich in blood and showed thick walls with ridges and pillars that protruded into the lung cavity, producing small alveolar protrusions. The features of the skin and lungs were similar to that of amphibians, testifying to the convergence of some tissue morphology in aquatic animals utilizing land as a cohabitat. J. Exp. Zool. 292:26–31, 2002. © 2002 Wiley-Liss, Inc.