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The aim of this project was to investigate some of the problems of solid-state fermentation. This technology is currently of limited use because of the complexity of traditional raw materials which make up the fermenting matrix and the inability to measure and control the fermentations. A novel, general purpose, cellulose based defined medium consisting of a liquid growth medium (75% w/w) absorbed onto a purified cellulose powder gave good growth of Asperigillus niger (IMI 17809) and Rhizopus oligosporus (IMI 205108), in a forced-aerated packed bed column fermenter. The defined medium allowed the monitoring of the fermentation by direct analysis of the matrix including substrate consumption, growth, and end product formation. Indirect monitoring of growth and activity was also performed by measurement of carbon dioxide production rate and by observing the changes in mixing and bed characteristics of the fermenter by gas tracer methods. Experiments were performed to determine the influence of temperature, inoculum size and substrate concentration on the growth and activity of A. niger. Using R. oligosporus, studies were made to compare its growth characterics with those of A. niger. The experimental data were analysed to determine correlations between microbial growth and activity with the consumption and production of materials. Only CO₂ production rates correlated well with growth, while substrate consumption and product formation did not. This is in contrast to liquid culture experiments with these organisms, where substrate uptake was related to growth. Microbial activity in the matrix could also be correlated to physical changes of the bed, as indicated by the tracer responses. The nature of the matrix (e.g. packing density) was an important consideration when applying the technique.