Growth, energy content, ingestion and respiration rates and energetic efficiencies were measured in the larvae of two marine fish (Solea senegalensis and Sparus aurata) whose eggs have a similar diameter (approximately 1mm) and energy content (approximately 1J), but whose larvae reach a quite different mass after the first month of life. Experiments were carried out in populations reared under the same conditions in the laboratory during the first month after hatching. Solea senegalensis grow from hatching to the start of metamorphosis (approximately day 14) at twice the rate of Sparus aurata (specific growth rate for Solea senegalensis 0.25 microg day(-1); specific growth rate for Sparus aurata 0.12 microg day(-1)). The tissues in Solea senegalensis larvae accumulated energetic reserves that were used during metamorphosis, which occurred during the third week after hatching. Ingestion and respiration rates differed in the two species during the experimental period. Although Solea senegalensis continued eating during metamorphosis, the specific ingestion rates decreased during the process. Nevertheless, no great differences in specific ingestion rates and rates of oxygen consumption were observed when comparing the same larval mass range. Larvae of both species showed an allometric relationship between respiration rate and biomass. The energetic efficiencies calculated in the present study denoted different metabolic patterns in each species. In Solea senegalensis, the energy used for growth increased progressively during the larval (pelagic) period and then, from the first signs of metamorphic transformation, remained almost constant. In this species, the energy allocated to growth was greater than that allocated to metabolic processes. Sparus aurata invested less energy in growth than in metabolic processes and displayed a constant energy allocation throughout the experimental period. During the first month after hatching, Solea senegalensis always allocated more energy for growth than did Sparus aurata.