Acid phosphatase was recovered from biomass of a Citrobacter sp. in two isoforms designated as CPI and CPII. Although similar, the enzymes had different stability properties which would relate to their suitability for biotechnological processes to remove heavy metals via enzymatically mediated precipitation as insoluble metal phosphates. The proportions of CPI and CPII extracted varied according to the Citrobacter strain and growth regime. Enzyme overproduction (mainly as CPII) used high-activity cells grown in batch culture whereas use of glycerol-limiting continuous culture gave initially mainly CPI and Inter predominantly CPII. This change corresponded to a fall in the total specific activity from 2,000-3.000 units to less than 600 units Initial batch studies showed increased phosphatase production where growth was limited by the degree of aeration. Further tests using carbon-limiting continuous culture suggested that the total enzyme activity was influenced by a combination of the dissolved oxygen concentration and the pH. These observations are consistent with a multifactorial regulation of the acid phosphatase gene phoN by the PhoP/PhoQ sensor/regulator system which is known to govern responses to stress in the related organism Salmonella typhimurium