Biocellion is a software framework for computer simulation of largescale multicellular biological systems portable to a wide range of parallel computers. Biocellion enables biologists who can model a single cell's behavior as a function of its environment to simulate colonies, communities, or organisms having hundreds of millions or more cells faster and with higher fidelity than with any other software of which we are aware. Biocellion itself is merely the computational framework: all domain specific knowledge is encapsulated in user models. This separation of concerns is an important attribute of Biocellion, providing generality across an increasing range of application domains without users needing any expertise in high performance parallel computation. The inventive features are integrated into the implementation of this framework, which manages the mapping of application parallelism to platforms ranging from laptop to supercomputer. Biocellion overlaps communication with computation, distributes application load to processors to achieve load balance, and exploits data locality to maximize throughput.