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Actively growing Escherichia coli C600(pBR322), immobilized within the macroporous matrix of asymmetric-wall hollow-fiber membranes, has been propagated to extremely high densities, typically more than 10 12 cells/mL of accessible void volume, in some regions cells accounting for nearly 100% of the available macrovoid volume forming a tissue-like mass. Production rates of β-lactamase, an enzyme used as an indicator of the culture's biosynthetic potential, remained at high and relatively stable levels for more than three weeks of continuous operation, and effluent supernatant enzyme activities attained 25% of the accumulated level measured in a 24-h shaker-flask culture. Based on the accessible void volume within the fiber wall, the β-lactamase productivity was independent of the specific asymmetric membrane used. On a per cell basis, however, cells cultured using hollow-fiber membranes were only 10% as productive as those in the shaker-flask culture, possibly due to the high packing density or culture aging. By contrast, the hollow-fiber bioreactor was 100 times more productive than the shaker-flask culture on a reactor-volume basis, primarily as a consequence of the high cell densities. Reactor productivity was dependent on the number of cells in the reactor, suggesting that reactor performance was kinetically controlled and not mass transport limited Immobilisation d'E.c. C600(pBR322) utilise comme microorganisme modele sur des membranes asymetriques de fibres creuses, et etude de la production de β-lactamase. La production de cet indicateur du potentiel biosynthetique de la culture, reste a un taux eleve et relativement stable pendant plus de 3 semaines de processus continu. Ce bioreacteur se revele 100 fois plus productif que la culture en flacon agite, du fait en particulier de la forte densite cellulaire