Osmoregulation was studied in the zoeal stages I to III, the megalopa, juvenile crab stages I and II, and in adults of the grapsid crab Armases miersii. The larvae hatch and develop in tropical supratidal rock pools, where ample variations of salinity occur. To cope with this harsh environment, the capacity for osmoregulation is well developed at hatching, and becomes further accentuated in the larvae and juveniles. All zoeal stages hyper-regulated at low salinity (5 to 26 PSU), but at higher salinity (33 to 44 PSU), they were hyper- osmoconformers. The type of osmoregulation changed in the megalopa stage to a hyper-hypo-regulation pattern. While the hyper-osmoregulatory capacity increased gradually throughout postembryonic development from hatching to adult, the hypo- osmoregulatory capacity increased from the megalopa to the adult. The ontogenic acquisition of osmoregulation was faster for hyper-than for hypo-regulation: 85 and 41% of the adult capacity, respectively, was acquired in the crab I stage. These findings confirm that important physiological changes occur at metamorphosis. A correlation is established between the osmoregulatory ability of each developmental stage and its salinity tolerance. The ecological implications and the adaptive and evolutionary significance of osmoregulation in early Life-history stages of A. miersii and other aquatic crustaceans are discussed.