Growth, hemolymph osmolality, tissue water, ribonucleic acid (RNA)/deoxyribonucleic acid (DNA) ratio, protein-bound amino acids and free amino acids (FAA) of Macrobrachium nipponense were investigated, after they were acclimated to 0x, 7x, 14xand 20xfor 14 days. Growth was significantly (P< 0.05) influenced by salinity. The highest weight gain of prawns (P<0.05) was achieved at a salinity of 14x. The RNA/DNA ratio, total protein-bound amino acid (TAA) and FAA concentrations in the muscle of prawns increased by hyperosmotic stress. The osmolality of prawns haemolymph increased slightly in salinities from 0xto14x. A sharp increase in haemolymph osmotic concentration occurs between 14xand 21x, the isosmotic point being reached at 15x (450 mOsm). It is noted that the relationship of total FAA concentration in muscle and salinity in the range of 0–20xrevealed a positive linear correlation. The correlation coefficient (R2) is 0.98. During adaptation, the FAA pool (mainly glycine, alanine and proline) of muscle seems to be directly related to osmoregulation, while the percentage change in alanine in prawns transferred from freshwater to diluted seawater (20x) was most pronounced (116% approx.). These results are compared with those of M. rosenbergii. © 2004 Elsevier B. V. All rights reserved.