The growth habitat of most filamentous fungi is complex and displays a range of nutritional, structural, and temporal heterogeneities. There are inherent difficulties in obtaining and interpreting experimental data from such systems, and hence in this article a cellular automaton model is described to augment experimental investigation. The model, which explicitly includes nutrient uptake, translocation, and anastomosis, is calibrated for Rhizoctonia solani and is used to simulate growth in a range of three-dimensional domains, including those exhibiting soil-like characteristics. Results are compared with experimental data, and it is shown how the structure of the growth domain significantly influences key properties of the model mycelium. Thus, predictions are made of how environmental structure can influence the growth of fungal mycelia. (C) 2008 The British Mycological Society. Published by Elsevier Ltd. All rights reserved.