During its flyby of Jupiter in 1974, Pioneer 11 passed through a region in which low-energy (~1 MeV) trapped protons had been exposed to absorption by Amalthea (L≂2.5) only ~5 hours earlier. In passing through this region, a brief (~10 s duration) decrease was observed in the counting rate of ~1-MeV protons measured by the University of Chicago charged particle instrument. We interpret this decrease as a transient signature of the recent absorption of the proton flux by Amalthea. No such clear effects were observed in fluxes of electrons (E≥3.4 MeV) or high-energy nuclei (Z>5, E>70 MeV/nucleon) at any crossing of Amalthea's orbit. These observations place an upper limit on the radial diffusion coefficient, &kgr;, for ~1-MeV protons at the orbit of Amalthea of 2¿1010 R2J/s. By comparison with diffusion coefficients derived from absorption effects by Io (L≂6), this limit requires that if the radial diffusion coefficient between L≂2.5 and L≂6 varies with L as L″, then n≥5, whereas diffusion driven by atmospheric neutral winds would predict &kgr;∝L3 for low-energy protons. Our results are consistent with diffusion driven by fluctuating electric (&kgr;∝L7--10) or magnetic (&kgr;∝L10) fields.