The growth ofRhizopus oligosporus on a model solid substrate consisting of cassava starch and other nutrients embedded in a kappa-carrageenan matrix was followed microscoplcally. There were two distinct growth phases: (1) germination of spores and development of mycellum to form a loose network over the whole substrate surface; (2) increase in the density of the mycellum. Although some penetrative hyphae were produced, the biomass was malnly restricted to the surface. Consequently, starch utilization was greatest near the surface. Based on these observations a descriptive model for the growth ofRhizopus ollgosporus on the model substrate was proposed. The five steps are: (1) release of glucoamylase by the mycelium; (2) diffusion of the glucoamylase to the starch; (3) hydrolysis of the starch by the glucoamylase, releasing glucose; (4) diffusion of glucose; (5) absorption of glucose by the mycellum at the surface to produce new blomass.