Problems of environment heterogeneity and interoperability between different mobile computing platforms are among the most urgent to address by the research community. This paper proposes an environment, which supports migration of mobile code developed using heterogeneous languages. Current developments in the area of mobile code and distributed mobile computing systems are discussed and placed within the context of the proposed system. The architecture behind the proposed Portable Environment Small Operating System (PESOS) provides an open platform for distributed persistent personal agents written using a variety of languages. Additionally, PESOS introduces and discusses the concept of a uniform migration wrapper consisting of portable file systems (PFS) within which applications and data are embedded.