Object-oriented construction of parallel PDE solvers. An object-oriented approach is taken to the problem of formulating portable easy-to-modify partial differential equation (PDE) solvers for realistic problems in three space dimensions. The resulting software library, Cogito, contains tools for writing programs to be executed on MIMD computers with distributed memory. Difference methods on composite, structured grids are supported. Most of the Cogito classes have been implemented in Fortran 77, in such a way that the object-oriented design is visible. With respect to parallel performance, these tools yield code that is comparable to parallel solvers written in plain Fortran 77. The resulting programs can be executed without modifications on a large number of multicomputer platforms, and also on serial computers.par The uppermost level of abstraction in Cogito decouples the numerical method from the PDE problem. The validity of these tools has been preliminarily demonstrated with a C++ implementation for one-dimensional problems.