  We study the twist-three fragmentation function contribution to the single
transverse spin asymmetries in inclusive hadron production in pp collisions,
pp->h+X. In particular, we evaluate the so-called derivative contribution which
dominates the spin asymmetry in the forward direction of the polarized proton.
With certain parametrizations for the twist-three fragmentation function, we
estimate its contribution to the asymmetry of pi0 production at RHIC energy. We
find that the contribution is sizable and might be responsible for the big
difference between the asymmetries in eta and pi0 productions observed by the
STAR collaboration at RHIC.
