The planktonic larval instars (zoea I, II, megalopa) of the spider crab Hyas araneus (Crustacea, Decapoda, Brachyura) were reared under constant conditions (12°C) in the laboratory. Rates of feeding (F), growth (G), respiration (R), and ammonia excretion (U) were measured in short intervals of time, and their changes during larval development were expressed as functions of age within each moult cycle. Energy values were calculated from uptake and accumulation of carbon, and from oxygen consumption and nitrogen loss, respectively. The regression equations describing temporal changes in these parameters were inserted in a simulation model, so that age-related variation patterns in the uptake and partitioning of nutritional energy could be analysed in relation to developmental processes such as the course of each larval moult cycle. The energetic traits of the two zoeal instars are much alike, whereas the megalopa reveals quite different characteristics. The time of maximum F shifts from the middle (in the zoea I) to the beginning (in the megalopa) of the moult cycle. F decreases in all larval instars during the premoult phase. Instantaneous assimilation (A) and G rates run almost parallel in each instar, showing maxima in early postmoult. In the zoeal instars, A and G decrease rapidly during later postmoult and intermoult, and they remain rather constant, at a low level, throughout the premoult stages. The same pattern can be seen in gross growth efficiency (K1) which levels off from >60 to ca. 30 to 20%. Net growth efficiency (K2), in contrast, decreases gradually in these instars throughout the moult cycle (from ca. 80 to 40%), and assimilation efficiency (A/F) shows a minimum (ca. 50%) in late intermoult and early premoult. In the megalopa, G, K1 and K2 decrease constantly throughout the moult cycle, becoming negative from early premoult (R > F). As a consequence, biomass and energy content of the megalopa reach maxima in the middle of this instar and then decrease, prior to metamorphosis. R shows an increasing trend during all larval moult cycles, with linear patterns in the zoeal instars and a cyclical one in the megalopa. U follows in all instars a bell-shaped variation pattern, with a maximum in early premoult. Excretory losses, however, constitute only an insignificant part of the entire energy budget (2 to 5 % of A). In the cumulative budgets, all parameters (F, G, R, U) increase from the first to the last larval instar. K1 and K2, however, decrease in the same order, and increasing portions of total G are lost as cast exuvia (9, 13, and 35 %, respectively). Significant changes in energy partitioning were found both during individual moult cycles and between different larval instars. These changes reflect developmental processes such as tissue growth, morphogenesis and, in the megalopa instar, the transition from pelagic to benthic life style prior to metamorphosis to the juvenile crab.