This experimental study examines the evolution of turbulence across an axial compressor and its modification by semicircular axial casing grooves (ACGs) at the pre-stall and near the best efficiency (BEP) flowrates. The turbulence is highly anisotropic and spatially inhomogeneous, with each normal Reynolds stress component evolving differently. Most of the observed trends can be explained by examining the dominant production rate terms. At the pre-stall flowrate, the turbulence increases significantly upon entering the rotor with peak RMS values of axial velocity fluctuations reaching as high as 71% of the mean axial velocity. The region with elevated turbulent kinetic energy (TKE) covers 30% of the outer span near the rotor leading edge, expanding to 50% near the trailing edge. While the TKE in the outer span decays rapidly in the stator, the local turbulence production persists in the stator blade boundary layer. By stabilizing and homogenizing the flow, the ACGs reduce the turbulence production, hence the TKE, in the rotor and the stator. The only exception is an increase in turbulence in the region dominated by groove–passage flow interactions. Near BEP, the TKE is much lower everywhere, except for the region influenced by the outflow from grooves. Downstream of the rotor and the stator, the turbulence level with or without ACGs are similar. The large variations in the magnitude and even the sign of the measured eddy viscosity highlight the extreme non-equilibrium conditions over the entire machine, questioning the fundamental assumptions of local equilibrium in eddy viscosity-based Reynolds stress models.