The use of CT scans in palaeontology has a long history. Most X-ray CT scans of fossils are carried out on previously prepared specimens and seldom on unprepared blocks of fossils. Here we report the use of a standard medical X-ray CT scanner to detect vertebrate and invertebrate fossils inside limestone blocks as an aid to subsequent preparation. The results were largely successful, with low-resolution images and radiodensity thresholds which nevertheless created sufficient contrast for identification of objects and their location inside blocks of limestone, thus optimizing the allocation of time and resources for palaeontological preparation. We conclude that the use of medical X-ray CT scanners for an initial visual inspection of limestone blocks for the presence of below the surface fossils is possible, cost effective and reliable. In addition, it allows the original raw data to be preserved as a digital object. The advantages of making use of standard medical X-ray CT scanners to facilitate palaeontological preparation under logistic or budgetary limitations is becoming more and more apparent.