Lipase production by Pseudomonas aeruginosa (strain MB 5001) was enhanced 6.6-fold by developing a fed-batch fermentation process. In batch culturing, lipase synthesis occurred during decelerated cell growth. Building on this information, lipase production was increased by feeding a balanced solution of glucose and ammonium chloride (carbon to nitrogen ratio of 4.13). Best performance was achieved when feeding was initiated during mid growth phase, prior to lipase biosynthesis. This lipase fermentation process was found to be unusually sensitive to dissolved oxygen tension, requiring cultivation of the microorganisms initially under dissolved oxygen limiting conditions followed by non-limiting dissolved oxygen conditions (lipase production phase). These process improvements yielded rapidly to scale up from laboratory bioreactors (23-l) to pilot plant bioreactors (1,900-l).