We examined the effects of varying water temperature (5, 12, 20 °C), ration size (ad libitum, and 3–4% of dry body weight per day), and injection of bovine growth hormone, on the growth characteristics of the epaxial muscle in laboratory-reared rainbow trout. Although the experimental conditions resulted in differences in growth rate, condition factor (K = W/L3 × 102) and dry weight, we found that, below a fork length of 18–20 cm, an increase in the cross-sectional area of the epaxial muscle occurs mainly by the addition of new fibres. However, between 20 and 25 cm the increase evidently results mainly from an increase in diameter of existing fibres. The relationship between fibre diameter frequency and fork length is characterized, and the implications of this relationship in limiting growth of this species is discussed.