1. Dry weight (W), carbon (C), nitrogen (N), hydrogen (H), total protein (Lowry), and total lipids were measured in larval decapod crustaceans reared in the laboratory with Artemia sp. nauplii as food.2. During complete larval development from hatching to metamorphosis, accumulation of biomass varied significantly between different species and biomass fractions, with a minimum C gain of 139% observed in lobster (Homarus gammarus) and a maximum of >7000% in swimming crab (Liocarcinus holsatus) larvae.3. Within indivdual moult cycles, up to >200% can be gained. Developmental changes (related to stages of the moult cycle) are responsible for most of the variability in elemental and biochemical composition of decapod larvae.4. Elemental or biochemical body composition (in % of W) does not change significantly with W in most decapod taxa. Only the Anomura showed a significant decrease in N and an increase in the C/N ratio with increasing W.5. Statistically significant differences were observed in the average (% of W) body composition of larvae belonging to different infraorders of the Decapoda. The most conspicuous differences occurred in the protein fraction, with an average value of 43% in the Caridea and only 30-32% in all other groups.6. There is an increasing tendency in the C/N ratio (indicating an increasing lipid/protein ratio) from primitive to advanced taxa, in the sequence: Caridea, Astacidea, Anomura, Brachyura. It is interpreted as a possible evolutionary trend towards an increasing degree of lecithotrophy in the Decapoda.7. Regression equations are given for conversions of W to CHN, N to protein, and C to lipid (the latter only for Brachyura).