An experimental investigation of shear localization at the tip of a notch in several material states of 300 maraging steel is reported. Side-impacted edge-notched plates were tested in the under-aged, peak-aged, and over-aged conditions, and the critical impact velocity above which shear localization occurs at the notch tip is reported for a notch root radius of 175 mu m. It is found that the critical minimum impact velocity required for shear localization is independent of the ageing condition. Next, the propagation of shear localization in two of the three ageing conditions is recorded using high speed photography at framing rates of 480000 frames per second. It is seen that, within the uncertainty of the experimental method, shear localization initiates at approximately the same time and stress intensity factor for the two materials. A heavy dependence of the shear band propagation speed and final length upon the ageing condition is seen. For the conditions examined shear failure is seen to propagate at an average velocity of 1000 m s(-1) in peak-aged material and 300 m s(-1) in underaged material. The peak-aged material fails fully by shear while the shear failure in under-aged material arrests and is followed by tensile failure. This result demonstrates the effect of material strength, roughly independent of the strain hardening characteristics for these materials, on shear localization propagation. The effects of the specimen and projectile geometry on the results are examined qualitatively using elastodynamic finite element analysis of the stationary notch tip loading.