Aeration of the head space in a shaken ceramic membrane flask (SCM flask) capped with a cotton plug was found to be essential for increasing cell concentration and viability of aerobic microorganisms. Ventilation through the cotton plug cap was insufficient for satisfying the oxygen demand of aerobically growing tells in the SCM flask. Dissolved oxygen concentration (DO) in the culture supernatant of Escherichia coli in batch culture using the SCM flask dropped to nearly 0 ppm when the cell concentration reached 1 g/l, while CO2 concentration in the head space of the SCM flask increased quickly to nearly 20% owing to accumulation of the evolved CO2. In contrast, the oxygen transfer coefficient in a SCM flask shaken at 230 rpm was as high as 220 h(-1), compared to that in a stirred type jar fermenter. Consequently, aeration of the head space of the SCM flask was found to be sufficient for supplying oxygen by vigorous waves created in the culture broth due to the reciprocal shaking action. Using the SCM flask, E. coli cell mass reached 84 g/l in 40 h with aeration of the head space with oxygen-enriched gas. The SCM flask with aeration to the head space enabled an increase in the concentration and productivity of viable cells to be readily achieved by continuously replenishing the culture supernatant with oxygen in conjunction with the removal of the evolved CO2.