The concept of logic databases can serve as a clear and expressive foundation of various kinds of information systems. However, classical logic languages refer to a single database state, whereas in modern information systems it is necessary to take dynamic behaviour into account. For this purpose, several update languages have been proposed, among them [1,5,6,9,16,17,18,23], which allow to specify complex transitions from one database state to another. From the evaluation point of view, a complex state transition can and should be considered as a transaction. Up to now, the isolation property of transactions has been poorly addressed in the context of logic update languages, although it is an important problem even for classical, sequential transactions (see