This paper presents a global Garbage Collector (GC) integrated in a MultiDataBase System architecture which preserves DataBase Management Systems (DBMS) autonomy. Each DBMS is supposed to have its own local GC and none assumption is made on the behavior of the local GC. There is no interaction between the global GC and the local GC. The global GC proposed is an adaptation of reference listing combined with a reverse mark and sweep technique. It has the following properties: it is incremental and requires few interactions with transactions; the reverse mark and sweep technique is able to detect dead object cycles that are frequent in a DBMS context; it is able to collect objects without accessing the whole database and global synchronization of DBMS sites is not required. The Global GC works exclusively on entry and exit items without accessing global object cells stored in DBMS. Consequently it implies few I/O overhead for the DBMS.