Megalopa larvae of a spider crab, Hyas araneus, were reared in the laboratory under constant 12°C, 32S, with diatoms (Biddulphia sinensis) given as food. Oxygen consumption, nitrogen excretion, and growth were measured in regular intervals during the moult cycle. Growth was measured as change in dry weight, carbon, nitrogen, hydrogen, protein, and lipid. Estimates of energy accumulation obtained independently from either carbon or biochemical components gave similar values of growth. The present results are compared with earlier data measured in carnivorous (Artemia-fed) larvae (ref. 10). In larvae fed with B. sinensis, accumulation of biomass amounted to only ca 8% of that in the Artemia-fed group. Energy loss by excretion increased in carnivorous larvae during the first days, then it remained stable throughout the rest of the moult cycle. It was low in larvae fed with diatoms, but increased at the end of the moult cycle, reaching similar values as in the Artemia-fed group. Energy loss by respiration was slightly reduced in larvae fed with B. sinensis, reaching 82% of the values observed in Artemia-fed larvae. Larvae fed with diatoms had high O/N ratios (86-96) at the beginning of the moult cycle, followed by decreasing values (eventually reaching 14 in premoult). This indicates that initially lipids were used as a predominant metabolic substrate, later proteins. Larvae fed with Artemia sp. showed O/N ratios varying between 20 and 40, with minimum values in the middle of the moult cycle, indicating in this group chiefly protein catabolism during the intermoult period. Energy budgets for the two different nutritional conditions were calculated. Net growth efficiency (K2) were 13.8 and 1.5 % in larvae fed with Artemia nauplii or B. sinensis, respectively.