In a model system for solid-substrate cultivation of Rhizopus oligosporus? it was observed that without pH control rapid acidification occurred, which inhibited the biomass production. With some degree of pH control, biomass production was improved significantly. Several buffering agents with different initial pH values were tested to find the optimum pH and the most suitable buffer for this model system. Between pH 4 and 6, the biomass-production rate was not significantly influenced by pH and that citric acid was the most suitable buffer. Addition of 5 g/l tryptone to a standard mineral medium increased both the maximum specific growth rate (mu(max)) and the maximum CO2-production rate, and strongly shortened the lag phase.