Planning adequate audit strategies is a key success factor in a posterion' fraud detection, e.g., in the fiscal and insurance domains, where audits are intended to detect tax evasion and fraudulent claims.A case study is presented in this paper, which illustrates how techniques based on classification can be used to support the task of planning audit strategies.The proposed approach is sensible to some conflicting issues of audit planning, e.g., the trade-off between maximizing audit benefits vs. minimizing audit costs.A methodological scenario, common to a whole class of similar applications, is then abstracted away from the case study.The limitations of available systems to support the identified overall KDD process, bring us to point out the key aspects of a logic-based database language, integrated with mining mechanisms, which is used to provide a uniform, highly expressive environment for the various steps in the construction of the considered case-study.