  A p-divisible group over a base scheme in characteristic p in general does
not admit a slope filtration. Let X be a p-divisible group with constant Newton
polygon over a normal noetherian scheme S; we prove that there exists an
isogeny from X to Y such that Y admits a slope filtration. In case S is regular
this was proved by N. Katz for dim(S) = 1 and by T. Zink for dim(S) > 0. We
give an example of a p-divisible group over a non-normal base which does not
admit an isogeny to a p-divisible group with a slope filtration.
