Larvae of the spider crab Inachus dorsettensis were reared in the laboratory at constant 12°C. Development lasted 8 to 10 d in the zoea I, 10 to 12 d in the zoea II, and 12 to 20 d in the megalopa stage. During this time, larval growth was measured in samples taken every 2 to 4 d as dry wt (W ), carbon (C ), nitrogen (N ), and hydrogen (H ); energy content (E ) was calculated from C. Biomass and energy (per individual) increased in each larval stage as a parabola-shaped function of age, which could be fitted by a power equation. C, H, and E show a higher percentage gain (relative to the initial values at hatching) than W or N, suggesting that proportionally more lipid than protein is accumulated during larval development. There are cyclical changes in the relative (per unit of W ) biomass and energy figures, corresponding to the larval moult cycles: immediately after each ecdysis all these values decrease, presumably due to rapid uptake of water and minerals, then they increase again due to tissue growth and remain high until the next moulting occurs. Cyclical changes in the C/N ratio suggest that proportionally more lipid than protein is accumulated during the initial (postmoult) phase of the moult cycle, followed by a period of balanced or protein-dominated gain during the intermoult and premoult phases. These patterns of growth and elemental composition observed during the complete larval development and in single moult cycles of I. dorsettensis are compared with those described in the literature for other decapod species. This comparison suggests a high degree of similarity in biochemical composition and growth characteristics of larval decapod crustaceans.