A multidatabase system (MDBS) is a system that supports interoperability among its component database systems (DBS), autonomously managed by component database management systems (DBMS). The authors propose a security control mechanism for MDBS based on multilevel security concept. The proposed security control mechanism provides the database administrators or security control officials of the MDBS and its component DBS with richer facilities for defining security constraints on the data items to be shared by the multidatabase system users. Furthermore, the security control mechanism can be implemented as a stand-alone front-end of component database systems so that it can be easily integrated into or decoupled from the MDBS