  The ac prenetration depth, \lambda_ac(T,H,f), has been measured in
superconducting UPt3 single crystals for H\parallel c and H\perp c in the range
f=0.01-1MHz and T=0.1-0.4 K. The contributions from bulk pinning and surface
pinning have been separated out to yield the flux-flow resistivity \rho_f(H).
With H\perp c, rho_f displays magneto-resistance at low field which agrees with
previous dc measurements and the characteristic scaling law of clean crystals
with anisotropic gap. When H\parallel c, the low-field \rho_f is three times
larger. We interpret this property as evidence for flux line with
unconventional core structure.
