Collaborative editing on large-scale ontologies imposes serious demands on concurrent modifications and con ict resolution. In order to enable robust handling of concurrent modications, we propose a locking-based approach that ensures independent transactions to simultaneously work on an ontology while blocking those transactions that might in uence other transactions. In the logical context of ontologies, dependence and independence of transactions do not only rely on the single data items that are modified, but also on the inferences drawn from these items. In order to address this issue, we utilize logical modularization of ontologies and lock the parts of the ontology that share inferential dependencies for an ongoing transaction. We compare and evaluate modularization and the naive approach of locking the whole ontology for each transaction and analyze the trade-o between the time needed for computing locks and the time gained by running transactions concurrently.