White, pink, red and deep red fibres, selected from a head muscle and from axial muscles of the perch, show significant differences in actin filament length, Z line thickness, Z line lattice space, myofibril girth, the percentages volume occupied by T system and terminal cisternae of the SR, and in the degree of T system/SR contact per sarcomere. In both muscles the degree of T system/SR contact decreases in the order: white, pink, red, deep red, which suggests a decrease of contraction velocity in the same order. The position of the T system (at the Z line or at the A/I junction) is related to the actin filament length. The actin filaments in the red fibres are appreciably longer than in the white, which suggests that the sarcomeres of the red fibres have a broader length-tension curve. The Z line thickness is positively correlated with the actin filament length and, in the white and the red fibres, negatively with the degree of sarcomere shortening. Thicker Z lines are suggested to allow greater sarcomere size (length or girth). The percentage volume occupied by mitochondria varies independently of the extent of membrane systems. The ultrastructural characteristics of the fibre types are in agreement with the functional roles as reported in literature.