A new approach to biofilm modeling based on cellular automata was described. Simple, local rules governing cell growth and erosion generated self-organized biofilm structures. In numerical experiments, the morphologies of simulated biofilms depended primarily on the thickness of the concentration boundary layer that controlled the external mass transfer. The bulk nutrient concentration had a weaker influence. For thin boundary layers, compact, dense base biofilms were generated. Thick boundary layers produced ramified, dendritic structures with higher porosities. For the same value of a critical bulk nutrient concentration, biofilm structures, generated in individual simulations, varied randomly between two very different outcomes: a dense, thick layer or almost complete erosion