The myotomal musculature of the common New Zealand smelt Retropinna retropinna from a lacustrine and a riverine population was investigated in an attempt to correlate differences in the fish's maximum body size and migratory habits with differences in muscle morphology and growth dynamics of their component muscle fibres. Based on measurements of cross sections through the fish at 70% fork length, a significant increase in red muscle percentage is recorded between pre- and post-migratory stages in the riverine population. Although lacustrine and pre-migratory riverine smelt possess similar white:red muscle ratios, the red muscle increase in post-migratory, riverine smelt is explained by the requirement for sustained swimming during upstream migration. The growth dynamics of red fibres also appear to be related to functional requirements such as sustained swimming. The slower rates of growth and smaller maximum size (dwarfing) exhibited by lacustrine smelt of basically riverine stock in comparison with diadromous, riverine smelt, are attributed to differences in white fibre growth dynamics. The recruitment of new fibres into the growing muscle mass ceases to be of significance at a shorter fork length in the dwarfed lacustrine form of smelt, with the subsequent result of larger mean diameters of white fibres at all fork lengths compared with those of the riverine form. Hypertrophy of white fibres gradually diminishes in both forms as the mean diameters of white fibres approach 100 $\mu $m. Considering that only fish from the wild were examined and that none of the data are derived from specimens subjected to forced swimming experiments in the laboratory, the results convincingly demonstrate the usefulness of the fish's muscle plasticity under natural conditions in relation to the environment and the behaviour of the fish.