  We define a differential Tannakian category and show that under a natural
assumption it has a fibre functor. If in addition this category is neutral,
that is, the target category for the fibre functor are finite dimensional
vector spaces over the base field, then it is equivalent to the category of
representations of a (pro-)linear differential algebraic group. Our treatment
of the problem is via differential Hopf algebras and Deligne's fibre functor
construction.
