  The massive six-dimensional (6D) superparticle with manifest (n,0)
supersymmetry is shown to have a supertwistor formulation in which its "hidden"
(0,n) supersymmetry is also manifest. The mass-shell constraint is replaced by
Spin(5) spin-shell constraints which imply that the quantum superparticle has
zero superspin, for n=1 it propagates the 6D Proca supermultiplet.
