A bug triage system is used for validation and allocation of bug reports to the most appropriate developers. An automatic bug triage system may reduce the software maintenance time and improve its quality by correct and timely assignment of new bug reports to the appropriate developers. In this paper, we present the techniques behind an automatic bug triage system, which is based on the categorization of bug reports. In order to obtain an automatic bug triage system we used these techniques and performed comparative experiments. We downloaded 1,983 resolved bug reports along with the developer activity data from the Mozilla open source project. We extracted the relevant features like report title, report summary etc., from each bug report, and extracted developer’s name who resolved the bug reports from the developers activity data. We processed the extracted textual data, and obtained the term-to-document matrix using parsing, filtering and term weighting methods. For term weighting methods we used simple term frequency and TF×IDF (term frequency inverse document frequency) methods. Furthermore, we reduced the dimensionality of the obtained term-to-document matrix by applying feature selection and latent semantic indexing methods. Finally we used seven different machine learning methods for the classification of bug reports. The best obtained bug triage system is based on latent semantic indexing and support vector machine having 44.4% classification accuracy. The average precision and recall values are 30% and 28%, respectively.