  It is conjectured by de Jong that, if $X$ is a connected smooth projective
variety over an algebraically closed field $k$ of characteristic $p>0$ with
trivial \'etale fundamental group, any isocrystal on on $X/W$ is trivial. We
prove this conjecture under two additional assumptions. Version 2: the main
change is an addendum. We prove that if $X$ is a connected smooth projective
variety over an algebraically closed field $k$ of characteristic $p>0$ with
trivial \'etale fundamental group, any infinitesimal isocrystal on $X/W$ is
trivial. To this aim we wrote some general facts on such infinitesimal
isocrystals over W which are missing in the literature.
