This paper presents a new framework for constructing mobile agents. The framework introduces the notion of agent hierarchy and inter-agent migration and thus allows a group of mobile agents to be dynamically assembled into a single mobile agent. It provides a powerful method to construct a distributed application, in particular a large-scale mobile application.To demonstrate how to exploit our framework, we construct an extensible and portable mobile agent system based on the framework. The system is implemented as a collection of mobile agents and thus can dynamically change and evolve its functions by migrating agents that offer the functions. Also, mobile agent-based applications running on the system can naturally inherit the extensibility and adaptability of the system.