Changes in the intracellular free amino acid (FAA) concentrations of Tigriopus brevicornis when subjected to hypoosmotic (5 psu); normal seawater (35 psu) and hyperosmotic (70 psu) conditions for a three day acclimation period were investigated. Proline, alanine and lysine were the major contributors to the intracellular FAA pool. Under hypoosmotic conditions, alanine was the dominant osmolyte with proline concentration at its lowest. This trend was reversed under hyperosmotic conditions. However, the total FAA pool was at its highest concentration under normal seawater conditions, with the total FAA concentrations being reduced during hypo- and hyper-osmotic conditions.