Abstract. Building on the work of Nogin, we prove that the braid group B 4 acts transitively on full exceptional collections of vector bundles on Fano threefolds with b 2 = 1 and b 3 = 0. Equivalently, this group acts transitively on the set of simple helices (considered up to a shift in the derived category) on such a Fano threefold. We also prove that on threefolds with b 2 = 1 and very ample anticanonical class, every exceptional coherent sheaf is locally free.