This thesis describes gas exchange, ion and water fluxes and nitrogen metabolism in the Canterbury mudfish, an amphibious fish, which was held in the laboratory in either water or air. The Canterbury mudfish, Neochanna burrowsius, is a swamp dwelling species endemic to New Zealand, which is now rare as its habitats are being destroyed by man. Passive and active exchanges in water or air were assessed using a number of methods, including measurement of gross fluxes, partitioning experiments and measurements of blood concentrations. The skin was found to be important for gas exchange and ammonia excretion in both water and in air. Ion and water fluxes through the skin were not unusually high. Indirect evidence points to a switch, on emersion, from protein to lipid catabolism. Even though the skin may be the site of more than half of the animals oxygen uptake, the gills and buccal epithelia were considered to be the primary gas exchange surfaces and by altering ventilation, the mudfish could compensate for reduced cutaneous exchange. Histological investigations of the structure of the skin, gills and buccal epithelium did not reveal any remarkable features that might be associated with emersion, though the epidermis of the skin possessed large numbers of mucous cells. The cutaneous exchanges, small body size, low metabolic rate and tolerance to fluctuating conditions were all considered important features of the overall emersion strategy. Physiological responses to emersion in mudfish more closely resemble those of intertidal marine amphibious fish species than those of tropical air-breathing species. Emersion tolerance was found to be well developed in most members of the galaxiid family. This ability, associated with cutaneous gas exchange, is discussed in relation to the family's marine ancestry, climbing ability and terrestrial spawning.