Abstract Single crystal YBCO samples frequently show a maximum in their critical current density j c( B) at large applied magnetic field B p (the `second peak', or `fishtail' effect). The origin of this technologically important effect is unclear. The decrease in j c at fields above the peak is probably related to plastic flow in the vortex glass. We have used SQUID based creep data to examine the second peak in both an optimally doped single crystal of YBCO and in a nominally identical sample which has been radiation hardened by fast neutron bombardment. Although the details differ, both samples exhibit a strong second peak. The creep data show a clear change in relaxation mechanism as B crosses the peak. We can also note that the fall in j c( B) for B> B p is in reasonable agreement with theoretical models for plastic flow depinning. Finally, we can use our data to extract values for the fundamental sample-dependent microscopic pinning parameter V 4. In both samples, a clear change in the functional form of V 4( B) takes place as B crosses B p, with BV 4 becoming constant with increasing field. Near the peak ( V 4)≈62 nm in each sample. This equality strongly suggests plastic flow.