  Let $S$ be the spectrum of a complete discrete valuation ring with fraction
field of characteristic 0 and perfect residue field of characteristic $p\geq
3$. Let $G$ be a truncated Barsotti-Tate group of level 1 over $S$. If ``$G$ is
not too supersingular'', a condition that will be explicitly expressed in terms
of the valuation of a certain determinant, we prove that we can canonically
lift the kernel of the Frobenius endomorphism of its special fibre to a
subgroup scheme of $G$, finite and flat over $S$. We call it the canonical
subgroup of $G$.
