We have investigated the injection and interplanetary propagation of low-energy protons that originated in a prompt solar particle event on July 20, 1981. The event was highly unusual in that solar flare protons could be seen from suprathermal energies (35 keV) to the energetic particle range (20 MeV). The observations were made in interplanetary space using the low-energy ion instrument (DFH) and ULEWAT instrument on ISEE 3. At the higher energies, the appearance of the time-intensity and time-anisotropy profiles in this event were characteristic of diffusive particle propagation: however, owing to extremely quiet interplanetary conditions, it was possible to observe solar particles down to proton energies well below previous studies. We were able to fit the observed time-intensity and time-anisotropy profiles over the entire energy range using a model based on the spherically symmetric Fokker-Planck equation including convection, diffusion, and adiabatic deceleration. From our our model fits we find that (1) the radial interplanetary diffusion coefficient K increases with heliocentric radius r as K=K0 rb, where b is close to 1; (2) the scattering mean free path for protons, &lgr;, increases with particle rigidity P as &lgr;∝P(0.22¿0.03) over the range between 9 and 160 MV; and (3) at Earth orbit, &lgr; values lie between 0.02 AU (for ~0.5-MeV electrons) and 0.17 AU (for protons, 10--20 MeV). Anisotropies in this event are long lasting (>48 hours) owing in part to an extended injection: the lower the energy, the longer the injection lasts. The extended period of injection appears to be evidence for diffusive coronal shock acceleration of protons during this event. Following the solar particle event maximum of intensity, a bidirectional streaming event was observed; during this period the particle anisotropies along with changes in the interplanetary magnetic field give evidence for the passage of a magnetic cloud by ISEE 3. ¿ American Geophysical Union 1990