  We have mapped out the spin dynamics in a stripe-ordered nickelate,
La$_{2-x}$Sr$_{x}$NiO$_{4}$ with $x \simeq 0.31$, using inelastic neutron
scattering. We observe spin-wave excitations up to 80 meV emerging from the
incommensurate magnetic peaks with an almost isotropic spin-velocity: $\hbar
c_s\sim 0.32$ eV \AA, very similar to the velocity in the undoped, insulating
parent compound, La$_{2}$NiO$_{4}$. We also discuss the similarities and
differences of the inferred spin-excitation spectrum with those reported in
superconducting high-$T_c$ cuprates.
