Free amino acid (FAA) contents of aquatic invertebrates as mollusca and crustacea were examinined in various tissues with different saline environment. Taurine, glycine and alanine increased by additional salinity in environment, and moreover, proline, glutamic acid and arginine increased a little by a sort of species and tissues. By these results, it seems that FAA plays generally a function of osmotic adjustment in poikilosmotic animals as aquatic invetebrates. As changes of FAA contents in tissue were a little in seawater (SW) acclimation of guppy, as it is, those role of osmoreguration is not obscure. For, both the decrease of taurine and alanine, and the increase of glysine and histidine were a little in tissue of guppy acclimated from freshwater (FW) to SW. And, the levels of other FAA were comparatively low and little different between those guppies acclimated in FW and in SW. Teleost fish as tuna, salmon, eel, carp, guppy were a typical homoiosmotic animals, and they have higher osmoregulatory function. So, it seems that a small amount of tissue FAA act indirectly to endocrinic and enzymic system for osmoregulation.

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