Throughout the larval development of a highly euryhaline crab, Neohelice (= Chasmagnathus) granulata, we studied effects of reduced salinities (15 and 25‰ vs. seawater control at 32‰) on larval dry mass (W) and biochemical composition (total lipid and protein). In the early zoeal stages (I, II), reduced salinity caused delayed moulting, while W and biochemical composition at ecdysis remained unaffected. This suggests that the capability of the Zoea I for hyper-osmoregulation (demonstrated in a previous study) mitigated potential effects of hypo-osmotic stress on biomass accumulation and biochemical composition, but did not prevent a developmental delay. After continued rearing at low salinities, later larval stages showed increasingly negative effects on growth. The Zoea IV, for instance, revealed at 15‰ significantly reduced W, lipid and protein contents compared to higher salinities, probably as a consequence of weak osmoregulatory capabilities in the zoeal stages II and III. Likewise, average daily rates of biomass accumulation (taking into account salinity- induced variations in development duration) were lower at reduced salinities. These negative effects on larval growth persisted throughout the Megalopa, in spite of a strong capability in this stage to hyper-osmoregulate. This indicates that the mitigating force of osmoregulation in the final larval stage was not strong enough to offset cumulative effects of a continuous exposure to osmotic stress lasting from hatching throughout the more stenohaline zoeal stages. Osmoregulation in N. granulata has implications for population dynamics in the field, where salinity conditions prevailing during the development in the plankton may influence larval condition and survival at metamorphosis, eventually affecting recruitment. © Inter-Research 2008.