In this paper we demonstrate the application of a holey-structured metamaterial lens for sub-wavelength imaging of defects in a metallic sample, in the ultrasonic regime. This type of lens, operating on the Fabry-Perot resonance principle, has earlier been demonstrated for super-resolution in the acoustic regime, and by the authors for nominal sub-wavelength resolution in the ultrasonic regime. Here we experimentally demonstrate a subwavelength imaging of an artificially created crack of size ?/15 in the aluminium sample: to our knowledge this is the highest resolution achieved in the ultrasonic regime. The subwavelength image obtained with the ultrasonic system incorporated with the holey-structured metamaterial is shown to compare favourably with corresponding results obtained using X-ray Computed Tomography (CT). © 2018 Electromagnetics Academy. All rights reserved.