S U M M A R YHomogeneous continuous-flow culture with some mechanical means for concentrating the biomass in the fermenter is termed a chemostat with feedback. The theory of the system has been reformulated and extended to include concentration of the biomass outside or inside the culture vessel by means such as centrifugation, filtration or gravity sedimentation. An important feature of the general system is the provision of two effluents, one of concentrated biomass and the other of diluted biomass. Experimentally, feedback was realized by filtration of one effluent stream. By this means, steady states were obtained over a wide range of flow rates; the values of biomass and growth-limiting substrate were in good agreement with the theory. The maximum biomass output rate of the chemostat was increased fourfold.

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Cytophaga johnsonii synthesized a polygalacturonate lyase which produced random cleavage of galacturonic acid polymers. No pectin methyl‐esterase or hydrolytic pectinase activities could be detected in cultures of the organism. Polygalacturonate lyase synthesis was inducible and also subject to repression by glucose and other compounds. Galacturonic acid was the most effective inducer; lower activities were obtained with citrus pectin, polygalacturonic and polypectic acids. Glucose repression of lyase synthesis was not alleviated by 5 mM‐adenosine‐3′.5′‐cyclic‐monophosphate. Enzyme production was growth‐linked and ceased when batch cultures entered the stationary phase. In steady‐state chemostat cultures lyase activity was maximal at a dilution rate (D) of 0.19 h‐1. Polygalacturonate lyase was both cell‐bound and free in the supernatant medium. The proportion of free enzyme increased throughout the batch growth cycle and in chemostat cultures over 70% of the activity was cell free at dilution rates below 0.05 h‐1.

Batch cultures of Agrobacterium tumejaciens were grown in a stirred reactor holding 400 litres of a minimal medium containing 20 g of sucrose litre-1. 3-Ketosucrose accumulated in the medium during the growth phase to reach a maximum concentration of 6 8 g litre-' at the point of sucrose exhaustion, 18-20 h after inoculation. The 3-ketosucrose produced was free from other sugars. The yield of 3-ketosucrose was influenced by initial aeration rate and inoculum size.