Introduction Microbial fermentation at a high cell concentration is an effective strategy for improving the productivity of useful solvents such as ethanol. Several methods are usually employed to achieve high cell concentrations in fermentation systems. These include: immobilization techniques (Chibata et al., 1986; Amin and Doelle, 1990), repeated-batch fermentation, and cell recycle (Scott, 1983) involving the use of separating devices such as hollow fiber filter modules, centrifuges and floc- culating strains of microorganisms (Ghommidh and Bu’Lock, 1988; Hughes et al., 1990). Hollow fiber membranes are self-supporting porous polymer tubes which have cylindrical channels. They offer one of the most cost effective ways of separating cells from culture media without cell damage. Recirculation velocity across the filter surface is of great importance to the overall filtration rate. Maintaining a high cross-flow rate reduces filter blockage allowing high flux rates. Hollow fiber filter was utilized for ethanol production by Zymomonas mobilis (Charley et al., 1983). Earlier work utilized a microporous cassette-type system for recycle (Beaton, 1980; Chang et al., 1981), however difficulties were encountered in long-term continuous cultures due to membrane and support screen blockage. Zymomonas mobilis (Doelle et al., 1993) is a promising agent for large scale production of ethanol because of its unusual physiological and biochemical properties (Viikari, 1988) as well as its high efficiency in ethanol production. In contrast to the exhaustive information available on yeast fermentation, much less work has been focused on the potential of Zymomonas mobilis for industrial ethanol production. Ordinary chemostat type cultures have inherent disad- vantages which makes them unsuitable for long term culture of anaerobic microorganisms. Unlike an aerobic culture, economic fermentation cannot be achieved by a reduction in the flow rate, as low sugar levels leads to nutrient starvation and loss of viability. The chemo- stat can therefore not be run close to the μ max . An arrangement of two or more fermenters in series however offers certain advantages over the use of a single chemo- stat, and theoretical models for these arrangements have been proposed (Pirt, 1975) but have not been used in experimental setups because of constraints with equip- ment. With biomass feedback maximum output rate of biomass and products in a fermenter can be increased in direct relationship to the concentration factor. The aim of the series of experiments was to develop a continuous culture system with high dilution coupled with high productivity without the risk of cell washout. Materials and methods Microorganism and growth medium Zymomonas mobilis NRRL-B14023 was grown on consisting of 100 g/l glucose, 10 g/l yeast extract, 1 g/l KH 2 PO 4 , 1 g/l (NH 4 ) 2 SO 4 , and 0.5 g/l MgSO 4 .7H 2 O. Composition of fresh medium feed was the same as the medium employed as growth medium. Inoculation and preculture Z. mobilis was first propagated at 30°C for 18 h without agitation by transferring 0.5 ml of culture from the 11111 2 3 4 5 6 7 8 9 10111 1 2 3 4 5 6 7 8 9 20111 1 2 3 4 5 6 7 8 9 30111 1 2 3 4 5 6 7 8 9 40111 1 2 3 4 5 6 7 8 9 50111 1 2 3111 Biotechnology Techniques, Vol 11, No 8, August 1997, pp. 537–541 © 1997 Chapman & Hall Biotechnology Techniques · Vol 11 · No 8 · 1997 537 Continuous culture systems involving Zymomonas mobilis and a hollow fiber filter module Zakpaa Domakyaara Hilary and Ayaaki Ishizaki* Lab. of Fermentation and Microbial Technology, Department of Food Science and Technology, Faculty of Agriculture, Kyushu University 46-09 Hakozaki, Higashi-ku, Fukuoka-shi-812, Japan Culture systems involving the use of one or two fermenters in series, with cell recycle employing hollow fiber filter modules to achieve high cell densities, were developed. Dilution rates of about 0.6 h –1 and ethanol productivity values of about 25 g/l h –1 , with residual glucose at 0.9 g/l, could be achieved using the system employing one fermenter.