Larvae of the hermit crab Pagurus bernhardus L. were reared under constant conditions in the laboratory from hatching through the first juvenile instar. In regular intervals of time (every 1 or 2 d), changes in biomass during individual moult cycles were measured in terms of dry weight (W), carbon (C), nitrogen (N) and hydrogen (H). Growth patterns in subsequent instars are described by regression equations. The megalopa does not eat and consequently loses biomass during its development, whereas all other instars gain biomass. Both instantaneous individual and instantaneous weight-specific growth rates decrease in general during the course of a moult cycle. As an exception, the megalopa was found to lose a constant fraction (ca 3 to 4 %.d-1) of its organic compounds. The C/N ratio indicates that this loss is primarily due to lipid catabolism. The term "secondary lecithotrophy" is proposed for non-feeding larval stages that develop with energy reserves accumulated by preceding feeding stages (in contrast to "primary lecithotrophy" of early stages that depend on yolk reserves from the egg). It is interpreted as an adaptation to an extremely specialized life style (here: life in a gastropod shell) requiring a particularly careful habitat selection before metamorphosis. As energetic costs of this adaptation, the megalopa loses ca one half of C and one third of N produced by the zoeal instars combined. Relative elemental composition (C, N, H as % of W) reveals a cyclic pattern with low postmoult and high premoult values during zoeal development. The megalopa, in contrast, shows decreasing and the juvenile rather constant values. Regressions are given that describe average relations between W, C, N and H, so that conversions are possible between different measures of biomass. Exuvial losses (in µg W, C, N, H individual-1) increase in successive instars in an exponential manner. As a percentage of late premoult matter or of the amounts produced during a given instar, exuvial losses of P. bernhardus zoeae are very low as compared to other decapod larvae, but these losses increase significantly in later developmental stages. Elemental composition shows that the exuviae of successive instars contain increasing amounts of inorganic matter (ash), including inorganic C (carbonate).