AMP nucleosidase [AMP phosphoribohydrolase, EC 3. 2. 2. 4] from A. vinelandii, which hydrolyzes AMP to form adenine and ribose 5-phosphate, has been purified. The purified enzyme is homogeneous as judged by its electrophoretic behavior. The enzyme is very unstable in the absence of divalent or trivalent anions such as sulfate, succinate or citrate, which effectively stabilize the enzyme. ATP and Mg2+ are absolutely required for the catalytic activity of enzyme. The Km values for Mg2+ and AMP are l×10−4 M and 5×l0−4M, respectively. The enzyme exhibits many of the characteristics of an allosteric enzyme. The rate-concentration curve with respect to the substrate (AMP) concentration is hyperbolic, whereas the curve with respect to ATP is sigmoidal. The enzyme activity is inhibited by GMP, IMP, phosphate, citrate and NH3 in an allosteric manner. In view of these data, AMP nucleosidase may be considered as an allosteric enzyme involved in regulating the degradation and interconversion of the purine nucleotides in A. vinelandii.