This communication describes the results of an investigation which was undertaken to clarify the contradictory effects of grain size variation on the tensile ductility and fracture toughness of aged 18Ni maraging steel and the possible underlying causes for the fracture mode transitions observed. Varying prior austenite grain size does not effect tensile ductility or fracture toughness. Increasing grain size does result in a decrease in tensile ductility. Fracture mode changes in the aged condition.