Ontogenetic changes in dry weight, elemental composition (CHN), and digestive enzyme activities (total protease, amylase) were studied during the moulting cycle of all larval instars (zoea I-II, megalopa; ZI, ZII, M) of the spider crab Maja brachydactyla Balss, 1922 reared in the laboratory. A gradual but significant increase in biomass was observed throughout the zoeal instars, followed by steeper growth in the megalopa. Maximum values were reached in moult stage D2. Digestive enzyme activities also increased significantly throughout larval development. The ZI showed a continuous increase in total protease activity during its moult cycle, suggesting that newly hatched larvae have a gradually increasing need to take up proteins to provide amino acids, which are prime materials for growth and development, and possibly to use them as a metabolic energy source. The largest variations in digestive enzyme activites in relation to the moult cycle were observed in the megalopa instar. After an initial increase in postmoult, enzyme activities decreased to low levels similar to those measured in the zoeal instars at intermoult, followed by another increase to a maximum level in premoult. Metamorphosis involves great physiological and behavioural changes, which could explain the large variations in the megalopa moult cycle. We therefore suggest that the moult cycle stages should be taken into account when nutritional condition indices are used to estimate the physiological performance of decapod crustacean larvae.