CHAPTER 1. The organisation and anatomical design of fish myotomal muscles. The literature on the anatomy, physiology and biochemistry of fish muscle is reviewed. CHAPTER 2. The effect of thermal acclimation on the metabolism of the swimming musculature of flounder (Platichthys flesus L.) Chapter 3. The effect of epinephrine administration on phosphorylase activity of the slow and fast muscles of flounder (Platichthys flesus L.) The results suggest glycogenolysis in flounder due to increased formation of phosphorylase a resulting from epinephrine activation of membrane bound adenyl cyclase. Chapter 4. Effects of hypoxia acclimation on anaerobic metabolism in the plaice (Pleuronectes platessa L.) There were no detectable levels of alcohol dehydrogenase activities, matched by the absence of ethanol pathway utilization. This may contribute to the failure of plaice to survive anoxia exposure. Chapter 5. Effects of acclimation to periodic anoxia exposure on the utilization of the ethanol pathway in goldfish (Carassius auratus L.). Chapter 6. Effects of acclimation to periodic anoxia exposure on the ultrastructure of goldfish (Carassius auratus L.) . CHAPTER 7. General Discussion. This section contains a general discussion of the biochemical responses to thermal acclimation and hypoxia. These responses precede ultrastructural changes which are stimulated to partially offset the adverse effects to normal homoeostasis. It includes a critical survey of current techniques and suggestions for future research.