The implications of multilevel security on database concurrency control are explored. Transactions are vital for multilevel secure database management systems (MLS/DBMSs) because they provide transparency to concurrency and to failure. Concurrent execution of transactions may lead to contention among subjects for access to data, which in MLS/DBMSs may lead to security problems. An abstraction of security models in terms of the transactions which they produce is presented. The notion of DC-Security, which identifies a class of covert channels that are caused by contention for access to shared data, is introduced. This notion is useful for evaluating the security of transaction schedulers. A framework for multilevel secure schedulers which allows analysis of a schedulers' security properties at the protocol level is presented. Necessary and sufficient conditions are developed for DC-Security in this framework and proved using noninterference. A wide range of schedulers is evaluated against these conditions.