The problem of ontology modularization is an active area of research in the Semantic Web community. With the emer- gence and wider use of very large ontologies, in particular in fields such as biomedicine, more and more application developers need to extract meaningful modules of these on- tologies to use in their applications. Researchers have also noted that many ontology-maintenance tasks would be sim- plified if we could extract modules from ontologies. These tasks include ontology matching: If we can separate on- tologies into modules based on the topics that these mod- ules cover, we can simplify and improve ontology match- ing. In this paper, we study a complementary problem: Can we use existing mappings between ontologies to facilitate modularization? We present a novel approach to modular- ization based on mappings between ontologies. We validate and analyze our approach by applying our methods to iden- tify modules for National Cancer Institutes Thesaurus (NCI Thesaurus) and Systematized Nomenclature of Medicine– Clinical Terms (SNOMED-CT).