The recombinant amylosucrase from Neisseria polysaccharea was used to glucosylate glycogen particles in vitro in the presence of sucrose as the glucosyl donor. The morphology and structure of the resulting insoluble products were shown to strongly depend on the initial sucrose/glycogen weight ratio. For the lower ratio (1.14), all glucose molecules produced from sucrose were transferred onto glycogen, yielding a slight elongation of the external chains and their organization into small crystallites at the surface of the glycogen particles. With a high initial sucrose/glycogen ratio (342), the external glycogen chains were extended by amylosucrase, yielding dendritic nanoparticles with a diameter 4-5 times that of the initial particle. A partial crystallization of the elongated chains induced a "shrinkage" of the nanospheres. The synthesis of linear alpha-(1,4) chains occurred simultaneously, yielding semicrystalline fibrous entities. All products displayed a B-type crystal structure. The kinetics of chain elongation and aggregation were thoroughly investigated in order to explain how the action of amylosucrase resulted in such different structures. These results emphasize the potentiality of amylosucrase in the design of original carbohydrate-based dendritic nanoparticles.