D-Amino acids have long been considered to be unique to microorganisms, which utilize some D-amino acids to construct the peptidoglycan layer of their cell wall. However, several D-amino acids have been detected in many organisms and even in mammals. Aquatic crustaceans and some bivalve mollusks contain a large amount of free D-alanine in their tissues. Along with the hyper-osmotic stress, several free amino acids have been known to increase as intracellular osmolytes in aquatic invertebrates, and the tissue levels of D- and L-alanine in crustaceans and bivalve mollusks were both increased during high salinity acclimation. These data suggest that D-alanine is also a major compatible osmolyte responsible for intracellular isosmotic regulation. Alanine racemase has been proven to catalyze the interconversion of D- and L-alanine in crustacean. A cDNA clone encoding alanine racemase has been isolated from the muscle and hepatopancreas of kuruma prawn Marsupenaeus japonicus for the first time in the animal kingdom. In this article, I would like to describe the physiological function and metabolism of D-alanine in kuruma prawn Marsupenaeus japonicus.