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To test if the TN = 17.7 K transition in URu2Si 2 is driven by a divergence of a magnetic order parameter we performed high-resolution eutron scattering. At the ordering wave vector the spin-wave energy collapsed and the susceptibility diverged as TN was approached. This confirms that the order parameter is the magnetic dipole, as shown by recent symmetry arguments and polarized neutron experiments [-1]. We also observe in-commensurate fluctuations, suggesting that competing temperature-dependent interactions may influence this weak-moment ransition. Speculation exists that the ordered phase that precedes superconductivity n the heavy-fermion systems URu2Si2 (TN = 17.7 K) [2] and UPt3 (TN = 5 K) I-3] might not be magnetic. The low apparent moments, 0.041ta.n_d 0.01/tB, respectively, have led to suggestions that the broken-symmetry Bragg peaks are quadrupolar [4] or a more exotic non-trivial phase [5]. To test these ideas we have made not only an elastic neutron scattering study [6] bu