Due to its platform-independent execution model, its support for networking, multithreading and mobile code, Java has given hope that easy Internet-wide high-performance network computing was at hand. Numerous attempts have then been made at providing a framework for the development of such metacomputing applications. Unfortunately, none of them addresses seamless sequential, multithreaded and distributed computing, i.e. the execution of the same application on a multiprocessor shared-memory machine as well as on a network of workstations, or on any hierarchical combination of both. In this paper we first identify four requirements for the development of such metacomputing frameworks. We then introduce Java/ / (pronounced Java Parallel), a 100 % Java library that provides transparent remote objects as well as asynchronous two-way calls, high reuse potential and high-level synchronization mechanisms. We also present the metaobject protocol (MOP) Java/ / is built on and describe a distributed collaborative raytracing test application built using Java//. ©1998 John Wiley & Sons, Ltd. 1.