For a countable ordinal we denote by C, the class of separable, reflexive Banach spaces whose Szlenk index and the Szlenk index of their dual are bounded by . We show that each C, admits a separable, reflexive universal space. We also show that spaces in the class C embed into spaces of the same class with a basis. As a consequence we deduce that each C is analytic in the Effros-Borel structure of subspaces of C[0, 1]. Instytut Matematyczny PAN, 2007.