The design of a microbioreactor is described. An optical sensing system was used for continuous measurements of pH, dissolved oxygen, and optical density in a 2 mt working volume. The K(L)a of the microbioreactor was evaluated under different conditions. An Escherichia coli fermentation in both the microbioreactor and a standard 1 L bioreactor showed similar pH, dissolved oxygen, and optical density profiles. The low cost of the microbioreactor, detection system, and the small volume of the fermentation broth provide a basis for development of a multiple-bioreactor system for high-throughput bioprocess optimization. (C) 2001 John Wiley & Sons, Inc.