Amylosucrase (sucrose:1,4-alpha-D-glucan 4-alpha-glucosyltransferase; EC 2.4.1.4) which mediates the transfer of the glucosyl moiety of sucrose to a growing alpha-1,4-glucan chain is a constitutive enzyme of Neisseria perflava. The products of enzymic action are insoluble glycogenlike polysaccharides and fructose, the latter being a competitive inhibitor of the enzyme (Ki=20 mM). The enzyme is extremely stable and appears to bind very tightly to its polymerized product. Properties of product-bound enzyme reflect those of the native complex.