The tropical crab Armases miersii breeds in supratidal rock pools, where great salinity variations occur. In laboratory experiments, all larval stages (zoea I-III, megalopa) and the first juvenile were studied as to their tolerance of acute salinity changes (5-55 parts per thousand, intervals of 10 parts per thousand) after preceding adaptation to ambient sea water (32-33 parts per thousand). Our observations suggest that some osmoregulatory capability exists from hatching. Survival was frequently higher and development faster at 15-25 parts per thousand than in sea water (35 parts per thousand), possibly indicating a phylogenetic adaptation of A. miersii to brackish water. Extremely low and high salinities (5, 45-55 parts per thousand) caused prolonged development durations and increased mortality rates. Tolerance of very low salinity, but not that of hypersaline conditions tended to increase during development. This suggests that late stages attain an increasing capability for hyper-osmoregulation in dilute media, but probably remain poor regulators in concentrated media. The megalopa and crab I were, in some experiments, less affected by chronic than by acute exposure to unfavourable salinities, suggesting that non-genetic resistance adaptation (acclimatization) had taken place during earlier development. During continual exposure to extreme salinities, however, effects of severe osmotic stress outweighed those of acclimatization. The range which allowed for successful development through metamorphosis widened with consecutively later change of salinity: 15-35 parts per thousand (exposure from hatching), 15-45 parts per thousand (from zoea II), 5-45 parts per thousand (from zoea III), 5-55 parts per thousand (from megalopa or crab I). It is concluded that tolerance of variable, predominantly low salinities in larval and early juvenile A. miersii has evolved as an adaptation to breeding in a physically unstable environment, where rapidly decreasing or continually low salinities are more likely to occur than hypersaline conditions. The degree of salinity tolerance in its early life-cycle stages shows that this species has reached, within the Grapsidae, a phylogenetically intermediate stage of adaptation to conditions in freshwater and terrestrial environments.