Developments in recombinant DNA technology are now allowing the controlled use of metabolic pathways to carry out a series of sequential organic chemical reactions using a single microbial catalyst. Biological processes of this type have significant industrial potential but in many cases still require the necessary biochemical engineering to translate them into a scalable process. Process synthesis is the systematic application of heuristics to the design of processes. Its application has enabled many chemical processes to be develped rapidly and feasibility assessed at an early stage in development. Here we apply such techniques to the area of multi-step microbial conversions (the use of metabolic pathway engineering to produce industrially useful chemicals). Biological and process engineering issues which require more research are identified. The use of the aromatic oxidation pathway serves as an illustrative example.