1. 1. The spider crab Hyas araneus L. was reared in the laboratory from hatching to metamorphosis. During each instar, the larvae were moult-staged, and their protein content, respiration rate, and activities of the digestive enzymes trypsin and amylase were measured in regular intervals of time. 2. 2. The patterns of change in these measurements of biomass and metabolism during the single moulting cycles and from instar to instar are described. 3. 3. Trypsin and amylase activities are linked by significant positive correlation, suggesting that they are regulated by a single factor. 4. 4. Two parallel regression lines with significantly different intercept values are obtained for the zoeal stages (combined) and the megalopa, respectively, when the activities of these enzymes are plotted against each other, with consistently higher amylase activities in the ultimate larval instar. 5. 5. Comparison with previous investigations on this species suggests that digestive enzyme activities are probably not related to feeding rates, however, they show positive correlations with respiration rates in the zoeal stages and negative correlations in the megalopa. 6. 6. Since H. araneus larvae are mainly carnivorous, the relatively high amylase activity does not correspond to the biochemical composition (low carbohydrates) of its food. It may be a phylogenetic relic from ancestor species with smaller, partly herbivorous larvae. © 1987.