Résumé : This article presents a case and field study on the debugging of a program that was to control an automatic machine tool installation. A programmer was observed full time for five weeks in his daily work, debugging the program written previously by a colleague. We chose to focus on a central process in debugging: the activity of comparison of pairs of representations, a reference and a target. For both, different types of representatations are constructed and compared consecutively, if necessary, before an error and/or its source are detected. These consecutive constructions and comparisons require increasingly complex information processing. Depending on the level in this progression, a diagnosis of the cause of the error is conducted to explain, in terms of program variables, the discrepancy between the observed (target) and correct (reference) system behaviour.