  We prove two flat descent statements for Artin n-stacks. We first show that
an n-stack for the etale topology which is an Artin n-stack in the sense of
HAGII, is also an n-stack for the fppf topology. Moreover, an n-stack for the
fppf topology which possess a fppf n-atlas is an Artin n-stack (i.e. possesses
a smooth n-atlas). We deduce from these results some comparison statements
between fppf and etale (non-ablelian) cohomolgies. This paper is written in the
setting of derived algebraic geometry and its results are also valid for
derived Artin n-stacks.
