Abstract. We consider the group of homotopy equivalences E(M ) of a simply connected manifold M which is part of the fundamental extension of groups due to Barcus-Barratt. We show that the kernel of this extension is always a finite group and we compute this kernel for various examples. This leads to computations of the group E(M ) for special manifolds M , for example if M is a connected sum of products S n × S m of spheres. In particular the group E(S n × S n ) is determined completely. Also the connection of E(M ) with the group of isotopy classes of diffeomorphisms of M is studied.The group E(X) of homotopy equivalences of a space X is the set of homotopy classes of homotopy equivalences X → X. The group structure is induced by mapcomposition. The group E(X), i.e. the group of automorphisms of the homotopy type of X, can be regarded as the homotopy symmetry group of the space X. In the literature there has been a lot of interest in the computation of such groups; compare for example the excellent survey article of M. Arkowitz [2]. This paper is concerned with the structure of E(M ) in case M is a closed, compact, oriented manifold, or more generally a Poincaré-complex. The computation of this group is an important step for the diffeomorphism classification of manifolds by surgery, [40]. The group E(M ) is also important for Cooke's theory [17] of replacing homotopy actions by actions. For a differential manifold M the group E(M ) is comparable with the group Π 0 Diff(M ) of isotopy classes of diffeomorphisms of M . In fact, via the J -homomorphism there is a striking similarity between these groups as is shown in § 10 below.Still there is little known on the group E(M ) in the literature; only very specific examples are computed, see [2]. This paper contains on the one hand general results on the structure of the group E(M ), see §1, . . . , §5; on the other hand our methods are used for explicit computations, see §6, . . . , §10.Let e