his paper describes the purification and properties of an enzyme present in Artemia larvae which hydrolyzes aminoacyl-tRNA by splitting the ester bond between the amino acid and the tRNA chain. The hydrolase has a molecular weight of 55 000 as estimated by gel filtration in Sephadex G-150, is maximally active in the presence of a divalent cation (Mg2+, Mn2+) and has a pH maximum at around neutrality. The enzyme has a wide substrate specificity, hydrolyzing with practically the same efficiency aminoacyl-tRNAs with the amino group free or substituted. This property distinguishes this enzyme from the widely distributed peptidyl-tRNA hydrolase and other more specific aminoacyl-tRNA hydrolases. The expression of the hydrolase during Artemia larval development is blocked by inhibitors of protein synthesis.