Chapter 1 General Introduction: The general introduction initially presents the major landmarks in muscle research of the last 3 millennia. The proteins of the contractile apparatus and their role in muscle contraction are described. There is then a description of how contractile proteins are known to alter through the expression of isoforms. Finally, a description of fish muscle and its fibre types is given, followed by the aims of the thesis. Chapter 2 Materials and Methods: An account is given of the materials and methods used throughout this thesis. This includes myofibril preparation and electrophoretic techniques. The techniques described are one and two dimensional polyacrylamide gel electrophoresis (PAGE), iso-electric focusing (lEF), peptide mapping, and one and two dimensional alkali-urea PAGE. The methods used to fix, stain and store the gels are then given, followed by the protocol used for Western blotting. Finally, the analysis of proteins bands is described. Chapter 3 Temperature and the plasticity of myofibrillar proteins during ontogeny in the Atlantic herring (Clupea harengus L.): Many aspects of development are influenced by temperature. The aim of this study was to investigate the effect of rearing temperature on the development and myofibrillar protein expression during ontogeny in the Atlantic herring (Clupea harengus L.). Chapter 4 The effect of body size on the myofibrillar protein composition of fast muscle fibres in the short horn sculpin (Myoxocephalus scorpius L.): The contractile properties of muscle vary with body length in fish. The aim of this study was determine if the proteins altered with body size in the short-horn sculpin (Myoxocephalus scorpius L.). Furthermore, could changes in protein expression be related to differences in contractile properties. Chapter 5 The myofibrillar proteins of Antarctic and sub-Antarctic fish: The myosins of Antarctic fish are specialised for function at low temperature. The aim of this study was to determine if the myofibrillar proteins present in Antarctic fish were highly conserved for function in this stable, low temperature environment. The variation in protein structure from myotomal fast muscles and the m. adductor profundis between five Antarctic fish species from two genera was compared with five sub-Antarctic species from four genera. The myofibrillar proteins of both the Antarctic and sub-Antarctic species showed a high degree of similarity between fish within the same genera. However, the isoforms present were considerably different between genera in both the Antarctic and sub-Antarctic species. Furthermore, the extent of the variation in protein isoforms between genera of the Antarctic fish was similar to that of the sub-Antarctic fish. This suggests that divergence in the tertiary structure of myosins from these species has occurred and that the Antarctic fish myofibrillar proteins are not highly conserved. Chapter 6 General Discussion: The major findings of the thesis are discussed in relation to the expression of myofibrillar proteins, with reference to further studies.