In this paper, we describe the Architecture Description Language (ADL) that we are defining for the design of agentbased systems. This aims at filing the gap between the analysis and design phases in agent-oriented methodologies. The analysis phase enables the description of the software architecture without any consideration of the execution environment while the design phase supplements the analysis output with descriptions related to the distributed environment in which the agents will run. The distributed environment we consider is OMG MASIF compliant mobile agent platforms. Our approach is to use the UML built-in mechanism of extension in order to provide a UML profile for the agent-based systems designers. Thus they can detail the software architecture of the system with information concerning the distribution aspects related to the execution environment. This work is a part of a project that deals with the development of a methodology for the construction of agent-based systems...