Hermit crab (Pagurus bernhardus) larvae were reared under constant conditions in the laboratory, and their growth and respiration were measured in short intervals (every 1 or 2 d) in terms of dry weight, CHN, and oxygen consumption, respectively. These measurements were converted to energy equivalents, in order to construct energy budgets for the successive larval instars. Instantaneous growth rates were constant in the zoea I, but showed a decreasing trend during all following zoeal moult cycles. The megalopa does not eat and consequently loses energy during its development. A decreasing C:N ratio indicates degradation of non-proteinaceous reserves (presumably mainly lipids) as an energy source during this stage. Individual respiration (R) increased in the first two zoeal instars as a linear function of time, then it showed a stepwise increase, with constant values during the zoea III and IV moult cycles. The megalopa showed maximum R during early postmoult, followed by significantly lower values during later stages of the moult cycle. In late premoult, however, a slight increase occurred in R, indicating enhanced metabolic activity with approaching metamorphosis. Moult-related increase in metabolic intensity of tissues was clearly shown also by a cyclic pattern in weight-specific respiration rate (QO2) during larval development. The average QO2 decreased in general in subsequent larval instars with average body weight. Computer simulations of bioenergetic changes suggest a decreasing trend in net growth efficiency (K2) during individual zoeal moult cycles, in particular during the zoea III and IV. When integrated energy budgets are calculated, however, the overall K2 values of the different zoeal instars are quite similar, with a somewhat higher value in the zoea I (0.58) than in the following three stages (0.44-0.47). Thus, energy partitioning does not reveal significant variation during zoeal development, whereas the megalopa deviates in all bioenergetic traits. As a consequence of secondary lecithotrophy, the ultimate larval stage respires ca. one half of total energy accumulated by the four zoeal stages combined. It is suggested that this peculiar energetic pattern in larval development of the hermit crab is an adaptation to its very specific habitat requirement at metamorphosis: the need for a gastropod shell.