1) The carpopodite extensor muscle of Carcinus maenas and Homarus americanus demonstrated regional localization of at least three types of fibre: fast phasic, slow tonic and intermediate. A specific area of the extensor, area 1, was identifiable in extensors from different animals and comprised fast phasic fibres in Carcinus and intermediate fibres in Homarus. When the M-C joint of the leg was held at a 90° angle, the sarcomeres in area 1 fibres were at rest length. Measurements of sarcomere length taken in the middle peripheral region were indicative of sarcomere lengths in other regions of the intermoult fibre. 2) The mechanism of growth of the fibres was determined in intermoult animals of increasing size. Fibres increased in length by the addition of sarcomeres and increased in width by an increase in the number of myofybrils. Myofibrils increased in size by the addition of thick and thin filaments in the characteristic lattice arrangement. The number of myofibrils may increase by myofibrillar splitting as has been previously described for vertebrate muscle. Fibre number remained constant with growth of the animal. There was a decrease in mitochondrial density with increase in fibre area. 3) Over ecdysis, fibres lengthened by addition of short sarcomeres to the exoskeletal region of the fibre. During the 3-4 days postecdysial expansion of the exoskeleton, these short sarcomeres increased in length. hypothesis that these short sarcomeres are precursor sarcomeres being added during ecdysis was investigated. When intermoult muscle was held in a stretched position for 2 weeks, there was a regional increase in the length of the muscle similar to the increases found over ecdysis. In contrast, fibre area did not increase over ecdysis. Instead, fibre area increased during the late postmoult and intermoult. The frequency distribution of myofibril size altered over ecdsysis; large myofibrils were more commonly present during premoult than during the immediate postmoult. Lateral stretch on the muscle during ecdysial expansion of the exoskeleton may be the stimulus for myofibrillar splitting. Possible satellite cells were identified in the skeletal muscle fibres-. vitro and jLn vivo radiolabelling techniques were adapted for the carpopodite extensor muscle of Carcinus maenas. The levels of free phenylalanine, total bound protein and phenylalanine bound in the muscle protein increased with size of the muscle. The mean in vitro rate of synthesis or FSR in the intermoult animal was 0.3% per day and the in vivo rate of synthesis at 1.24% per day. The FSR remained constant with size of the extensor muscle in intermoult animals. Fixatives may have an effect on the specific activity in the bound fraction of the muscle which is attributable to binding artifacts of the free amino acids. 5) The levels of phenylalanine in the free pool fluctuated during the moult cycle. The protein/wet weight ratio of the muscle also varied with the moult cycle. Increases in protein during the early postmoult supported other evidence for growth of the muscle over ecdysis. Synthesis rates were raised during the premoult and immediate postmoult period and remained slightly elevated during the late postmoult. These increases in rate corresponded to periods of muscle growth. 6) Autoradiographic techniques combined with measurements of bound specific activity revealed an increase in activity in the exoskeletal or cuticular region of the fibre corresponding to the region of new muscle synthesis over ecdysis. No differences in grain density were found between fast phasic and slow tonic fibres or between the central and peripheral region of the myofibril in intermoult muscle. Regional localization of grains was found at the light microscope level in the Z, I and B bands but no further evidence of this differentiatial labelling was found at the electron microscope level.