yA mechanistic model has been developed to describe mass transfer and growth kinetics in filamentous fungi on solid substrate before branching takes place. The model accounts for substrate diffusion in the solid medium, uptake and release of substrate at the fungal wall membrane by a carrier, and formation of wall precursors coupled to mass transport by diffusion and convection inside the hypha. Model simulations for a range of substrate concentration showed good qualitative agreement with experimental data, when measured values of hyphal diameter and maximum length of a germ tube were employed. The model was used to evaluate the relative contribution of convective flow to the overall mass transport inside the hypha, to explore the magnitude of substrate concentration micro-gradients in the solid medium, and to assess the role of transport and kinetic parameters in the behaviour of hyphal growth.