The neotropical crab Armases miersii (Rathbun, 1897) breeds in supratidal rock pools, where great salinity variations occur. In laboratory experiments, all larval stages and the first juveniles were reared at six different salinities (5-55 PSU, intervals of 10 PSU). In five series of experiments, exposure to these conditions began either from hatching (Zoea I) or from the onset of successively later stages (Zoea II, III, Megalopa, Crab I). Growth was measured in terms of dry weight, carbon, nitrogen, and hydrogen content. At osmotically extreme conditions (5, 55 PSU), all stages showed minimum biomass accumulation; this was consistent with maximum mortality and longest duration of development (data presented in a separate paper). Successively later exposure to these salinities tended to reduce these effects. Lowest mortality and shortest time of development occurred generally at 15-25 PSU, indicating an optimum at moderately reduced salinities. This response pattern, however, was not congruent with that observed in growth. Biomass accumulation was initially maximum within a wide range of salinities (15-45 PSU), but in the Zoea II and III stages this range tended to narrow and to shift towards higher salinities (35-45 PSU). These trends reversed in the megalopa and Crab I, where maximum growth occurred again in a wider range and at lower salinities (15-35 PSU). The reduction of zoeal growth in moderately dilute media (15-25 PSU), which were optimal for survival and development, is interpreted as an energetic cost of hyper-osmoregulation, which begins already at hatching. 5 PSU caused hypo-osmotic stress, exceeding in the long term the larval capacity for hyper-regulation. Poor zoeal survival and growth at 55 PSU are interpreted as effects of hyper-osmotic stress. In the Megalopa and Crab I, reduced growth at salinities *35 PSU may reflect the energetic costs of hypo-osmoreguation beginning in these stages. Our data suggest that the physiological adaptations of larval and early juvenile A. miersii allowing for survival and development in a physically harsh and unpredictable habitat imply a trade-off with reduced growth, due to energetic costs of osmoregulation.