Growth of pure cultures of Geotrichum candidum or Penicillium camembertii at the surface of a solid medium was studied. Consumption of nutrients by the microorganisms growing at the surface of the gel induced their diffusion from the core to the rind. Particular attention was paid to the diffusion of glutamic acid, the nitrogen source, in relation to the growth of the microorganism at the surface. The growth kinetic has been described using the Verlhust model. The diffusion coefficient of glutamic acid in sterile culture medium was measured and found to be 0.55 cm(2) day(-1). With this coefficient and assuming that the glutamic acid consumption was partially linked to growth, the experimental diffusion gradients have been fitted. Good agreement was found between experimental data and the diffusion/reaction model. The glutamic acid diffusion cannot be assumed to limit growth, since noticeable amounts of this substrate always remained at the upper surface of the gel. (C) 2004 Society of Chemical Industry.