Muscle cellularity patterns in teleost fish have normally been investigated using animals reared under constant temperature conditions. In the present study, Danube bleak (Chalcalburnus chalcoides mento) were reared under two different rising temperature regimes (cold, 12-16 degrees C; warm, 18-20 degrees C) designed to mimic the natural conditions experienced by the fish in temperate freshwater environments. Samples were taken from both groups of animals at intervals during their development. Transverse sections at the level of the anal vent were examined using light and electron microscopy, histochemistry and immunohistochemistry techniques. Total cross-sectional area of red and white muscle, as well as fibre numbers and fibre cross-sectional areas of one epaxial quadrant per specimen, were measured. Analysis of fibre numbers and sizes indicated that white and red myotomal muscles each develop in a different manner. In white muscle, the initial growth phase is dominated by fibre hypertrophy, while the later larval growth phase also includes significant hyperplasia. Red muscle growth is mainly due to hypertrophy within the studied developmental period. The temperature regimes applied in the present study may modify the mechanisms of muscle growth in different ways. For white muscle, pre-hatching hyperplasia (i.e. proliferation of somitic white fibre precursor cells) is reduced under the cold regime whereas post-hatching hyperplasia is not. The inverse is true for white fibre hypertrophy. A similar situation is seen with red muscle except that post-hatching hyperplasia is low and refractory to temperature. Rates of increase in relative amount of red muscle appear to depend not only upon species and temperature but also upon whether the fish have been reared under changing or constant thermal regimes. These findings are discussed in relation to 'landmark' events of early ontogeny (hatching, onset of swimming, start of exogeneous feeding) and to their implications for future accurate interpretation of temperature effects on teleost developmental biology and functional ecology.