Program slicing has many applications such as program debugging, testing, maintenance, and complexity measurement. This paper concerns the problem of slicing concurrent object-oriented programs that has not been addressed in the literatures until now. To solve this problem, we propose a new program dependence representation named the system dependence net (SDN), which extends previous program dependence representations to represent concurrent object-oriented programs. An SDN of a concurrent object-oriented program consists of a collection of procedure dependence nets each representing a main procedure, a free standing procedure, or a method in a class of the program, and some additional arcs to represent direct dependences between a call and the called procedure/method and transitive interprocedural data dependences. We construct the SDN to represent not only object-oriented features but also concurrency issues in a concurrent object-oriented program. Once a concurrent object-oriented program is represented by its SDN, the slices of the program can be computed based on the SDN as a simple vertex reachability problem in the net.