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The temperature and magnetic-field dependence of activation energy U o (T,H) for creep has been studied in an HgBa 2 CuO 4ϩ␦ single crystal below and above the ''fishtail'' peak H sp (T). At constant temperature, U o (H) increases up to H sp / (T) and then for HϾH sp / (T) it decreases. H sp / (T) is located lower than the field H sp (T) deduced from the hysteresis loops. The decrease of the activation energy is probably indicative of a plastic-type creep of the vortex lattice. We demonstrate that the overall behavior of U o (T,H) at the second magnetization peak line H sp (T) in the phase diagram of HgBa 2 CuO 4ϩ␦ exhibits similarities with that observed in

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