This data set contains a 3D model geometry describing 600x500x400µm cardiac tissue with 531 myocytes and no other cell types or structures. It was artificially generated from a random network geometry using methods based on those described here, and remeshed using mmg3d version 5.8.0. The mesh is provided in two formats: Mmg format (.mesh) and openCARP format (all other files) The .mesh format is well described in the documentation of libMeshb, a C library that can read and write this format. It can be read by the Medit program to visualize and explore, and converted to other formats by meshtool and other software. The openCARP format can be read directly by the openCARP code. In both formats the tetrahedral elements are described by four vertex numbers and an integer tag. Odd tags identify intracellular volume and even tags extracellular volume. Each pair of tags (2n, 2n+1) identifies the space belonging to a cell. Especially on the boundaries of the mesh there are several extracellular domains without corresponding intracellular domains.