  Let $BS(1, n)=< a, b | aba^{-1} = b^n >$ be the solvable Baumslag-Solitar
group, where $ n\geq 2$. It is known that B(1, n) is isomorphic to the group
generated by the two affine maps of the line : $f_0(x) = x + 1$ and $h_0(x) =
nx $. The action on $S^1 = \RR \cup {\infty}$ generated by these two affine
maps $f_0$ and $h_0 $ is called the standard affine one. We prove that any
representation of BS(1,n) into $Diff^1(S^1)$ is (up to a finite index subgroup)
semiconjugated to the standard affine action.
