Let p be a prime. Let V be a discrete valuation ring of mixed characteristic
(0,p) and index of ramification e. Let f: G \to H be a homomorphism of finite
flat commutative group schemes of p power order over V whose generic fiber is
an isomorphism. We bound the kernel and the cokernel of the special fiber of f
in terms of e. For e < p-1 this reproves a result of Raynaud. As an application
we obtain an extension theorem for homomorphisms of truncated Barsotti--Tate
groups which strengthens Tate's extension theorem for homomorphisms of
p-divisible groups.