A practice shows that numerous frauds based on computer technology represent new and sophisticated form of modern criminal which can cause dramatic consequences, not only for individual business systems but also globally. Since this criminal form is realized through information systems, their audit, as a constituent part of necessary control measures, becomes indispensable nowadays. However, existing methods for information systems auditing are still in the process of development and at this moment no methodology exists. Consequently, basic objective of this work is to develop such computer assisted information system auditing methodology. Foundation of such methodology is application of Benford's Law which links other existing auditing methods. Analysis of literature shows that this field of research is still insufficiently investigated. Therefore, these issues are thoroughly studied, model of methodology is designed, specificmethodology is developed, a number of tests and other examinations areperformed, methodology is checked on representative sample and recommendations for further development of information systems audit activities and corresponding auditing software tools are presented. A field of research is financial industry (cross border payment system), because major frauds occurred in financial industry segment. These frauds were possible because lack of adequate controls which should have been checked by information systems audit.