Morphological changes are described in Japanese flounder Paralichthys olivaceus larvae and juveniles with emphasis on growth during the period of metamorphosis. Ontogenetic changes in condition factor and lipid, glycogen and protein levels were also analysed to determine the utilization of stored energy. Fish grew from 6(.)6 to 20(.)3 mm L-T (1(.)15 to 84(.)4 mg in mass, M) during the period from 11 to 40 days after hatching (DAH) at 19(.)3degrees C. Per cent specific growth rate per day (% G day(-1)) for wet mass was lowest during the metamorphic phase (21-30 DAH) compared to pre- (11-20 DAH) and post-metamorphic (31-40 DAH) phases. When L-T and M were expressed as M = a L-T(b), the b value was highest during the pre-metamorphic phase and lowest during the metamorphic phase. These findings indicate that the developmental changes that occur during metamorphosis of Japanese flounder are closely related to the growth pattern. Moisture, lipid and glycogen contents were also at the lowest level during metamorphosis compared to pre- and post-metamorphosis, which suggest that Japanese flounder use their energy reserves to accomplish metamorphosis due to an apparent decline in feeding during this period. (C) 2003 The Fisheries Society of the British Isles.