In this paper, we propose a new multiversion locking protocol for real-time database systems for providing multilevel security. Real-Time DBMS (RTDBMS) is a system that processes transactions with explicit timing constraints. Thus, it must satisfy not only logical data consistency but also timing constraints associated with transactions. There are increasing needs for security in many applications including real-time database systems. Multilevel security requirements introduce a new dimension to transaction processing in RTDBMS. In multilevel secure (MLS) database systems, a user's access is restricted based on the clearance of the user and the classification of data. There exist conflicting natures between timing constraints and security requirements. We propose a new protocol ensuring both two conflicting requirements. We first classify transactions according to their priorities and security levels. Next, the compatibility matrix and a new version selection algorithm for our protocol are presented. In addition, the conditions for satisfying relative temporal consistency are given.