  We have found a sharp transition at T_c2=350 mK in the vortex creep rate of a
single crystal of (U_1-xTh_x)Be_13 with T_c=523 mK (x=0.0275). For T<<T_c2, no
creep of vortices is observed in a time scale of 10^5 s, while for T_c2<T<T_c,
vortices creep at very high rates (30 % of decay from a metastable
configuration in the first 10^5 s at T=400 mK). The sharp transition occurs at
the same temperature at which the second jump in the specific heat appears in
these samples. Similar low levels of creep rates have been reported by us in
the low-T superconducting phase of UPt_3.
