TOP2 is a tool suite that aids users of parallel systems with distributed memory in porting existing sequential applications by supporting the separation of compute-intensive kernels of an application from the existing sequential code and providing a development environment for the parallelization of these code segments.In this scenario, the sequential and the parallel code are run simultaneously as a distributed application on both systems and automatically exchange context data between both components. Main features in this process are the provision of cross-domain message passing for the automatic distribution of program data from the sequential machine to the distributed memory system and the ability of on-line debugging of the parallelized code.The data distribution features of TOP2 are a subset of those defined in HPF Fortran and thus especially support algorithms on regular data structures exploiting data parallelism in the context of SPMD programming.The TOP2 User's Guide (Version 4.01) describes all features of TOP2 and its handling. New in Version 4.01 arethe support of multiple parallel program modules,the extended support of Fortran language extensions: include statements, Fortran 90 style inline comments,the support of multiple source input files,a new structure of the generated source code (all user-specific code in one file)