  We present elastic and inelastic neutron scattering data on LaSrFeO$_4$. We
confirm the known magnetic structure with the magnetic moments lying in the
tetragonal basal plane, but contrarily to previous reports our macroscopic and
neutron diffraction data do not reveal any additional magnetic phase transition
connected to a spin reorientation or to a redistribution of two irreducible
presentations. Our inelastic neutron scattering data reveals the magnon
dispersion along the main-symmetry directions [0 $\xi$ 0] and [$\xi$ -$\xi$ 0].
The dispersion can be explained within linear spin-wave theory yielding an
antiferromagnetic nearest-neighbour interaction parameter $J_{1}=7.4(1)$ meV
and a next-nearest neighbour interaction parameter $J_{2}=0.4(1)$ meV. The
dispersion is gapped with the out-of-plane anisotropy gap found at
$\Delta_{out}=5.26(2)$ meV, while evidence is present that the in-plane
anisotropy gap lies at lower energies, where it cannot be determined due to
limited instrument resolution.
