Bioreactor headspace pressurization represents an excellent means of enhancing oxygen mass transfer to a culture. This method is particularly effective in situations where stirring or vigorous aeration is difficult. Because it in itself introduces no undesirable hydrodynamic force, the proposed method is also attractive for cells susceptible to agitation and sparging. Experiments were first conducted in an ideal fermentor by sparging air into a sulfite solution free from extraneous microbial effects. An increased oxygen mass transfer rate resulting from pressurization led to a superior cell growth rate and a higher maximum cell density in both of the microbial systems studied: a bacterial (Escherichia coli) culture up to 2.72 bar and a fragile algal (Ochromonas malhamensis) culture with pressure programming. Applying pressurization increased the maximum dry cell weight from 1.47 g/L to 1.77 g/L in the E. coli culture and increased the maximum viable cell density from 4 x 10(7) cells/mL to 10(8) cells/mL in the algal culture. An additional advantage is that formation of undesirable products under oxygen limitation, e.g., acetic acid in the E. coli culture, can be suppressed. A significant (over 250%) improvement in the oxygen transfer rate can be achieved with existing fermentors with little modification as they are already designed to withstand reasonable pressure from autoclaving. This method is simple, clean, inexpensive, and easily implemented, and it can be applied alongside other existing methods of oxygen mass transfer enhancement.