Mole is one of the first Java-based mobile agent systems. It runs on an unmodified Java virtual machine on PCs under Windows 95/NT and on Workstations under several UNIX dialects. Earlier versions of Mole have provided a basic infrastructure for communication and migration of mobile agents. Version of Mole has been strongly revised and several requests and proposals from users of the earlier ver sions of Mole were integrated into the new release. In particular Mole supports communication between agent groups and concept of sessions. The infrastructure of Mole includes a resource manager, a direc tory service and a global naming scheme for agents. In order to support the design of agents, a graphical agent monitor allows to visualize the system behaviour as a whole or of a single agent in particular. Mole further provides a thread management unit to overcome some shortcomings of the Java virtual machine. Mole provides a simple means for installation and configuration of the system. This paper describes the architecture and infrastructure provided by Version 3.0 of Mole.