The spin wave dispersion in NiBr 2 has been studied by medium and long wavelength inelastic neutron scattering in the [1 1 0], [1 0 0] and [0 0 1] directions at 4.2 and 30 K, i.e. in the incommensurate helical and collinear antiferromagnetic phases. The values of the intralayer Heisenberg exchange constant J ij and XY anisotropy constant D at 4.2(30) K are J 01 0.379(1)(0.379(1)), J 02 0.0036(50)(0.0036(50)), J 03 - 0.105(5) (−0.105(5)), J′ - 0.0423(50)(−0.389(50)) D 0.0364(50)(0.0290(50)), where J′; is the interlayer exchange constant. In fitting the 4.2 K data account is taken of the co-existence of three equivalent domains and of intensity arising from ω( q) and ω( q ± k 0) where k 0 is the wavevector of the helix. In the low frequency region of the dispersion curve such peaks are resolved. The results reinforce the hypothesis that in zero-field the commensurate-incommensurate phase transition is driven by fluctuations.