1, which corresponds to local angular forms on the sphere bundle, and which survives to E n , to the Euler class of the sphere bundle. Since SO(n) is compact, the spectral sequence of the bundle collapses at E 2 , by the theorem. Thus d n = 0 and we obtain the following corollary:Corollary. The real Euler class of an oriented, flat, linear sphere bundle (structure group SO(n)) over a closed, smooth manifold is zero.We thus obtain a topological proof, without using the Chern-Weil theory of curvature forms, of a result closely related to the results of [Mil58, Section 4], which do rely on Chern-Weil theory.By [Smi77], there is a flat manifold M 2n with nonzero Euler characteristic for every n > 1. If the associated tangent sphere bundle of M, with structure group SO(2n), were flat, then the Euler class of such a sphere bundle would vanish according to the above corollary. We arrive at: