  Let \lambda be a partition of a positive integer n. Let C be a symmetric
rigid tensor category over a field k of characteristic 0 or char(k)>n, and let
V be an object of C. In our main result (Theorem 4.3) we introduce a finite set
of integers F(\lambda) and prove that if the Schur functor \mathbb{S}_{\lambda}
V of V is semisimple and the dimension of V is not in F(\lambda), then V is
semisimple. Moreover, we prove that for each d in F(\lambda) there exist a
symmetric rigid tensor category C over k and a non-semisimple object V in C of
dimension d such that \mathbb{S}_{\lambda} V is semisimple (which shows that
our result is the best possible). In particular, Theorem 4.3 extends two
theorems of Serre for C=Rep(G), G is a group, and \mathbb{S}_{\lambda} V is
\wedge^n V or Sym^n V, and proves a conjecture of Serre (\cite{s1}).
