Due to heterogeneity in modern computing systems, methodological and technological problems arise for the development of parallel applications. Heterogeneity occurs for instance at architecture level, when processing units use different data representation or exhibit various performances. At interconnection level, heterogeneity can be found in the programming interfaces and in the communication performances. In this paper, we focus on problems related to heterogeneity in the frame of interconnected clusters of workstations. In order to help the programmer to overcome these problems, we propose an extension to the multithreaded programming environment PM2. It facilitates the development of efficient parallel applications on such systems.