The suitability of olive mill wastewater (OMW) as a growth medium for lipase production was assessed using Penicillium citrinum NRRL 1841, a versatile strain capable of producing lipase on several OMW typologies differing widely in their chemical oxygen demand, polyphenol and total sugar contents. Lipase production by P. citrinum in OMW-based media was significantly stimulated by nitrogen addition, with ammonium chloride proving to be the most effective source. In contrast, the addition of vegetable oils did not significantly affect lipase production. On the basis of these results, lipase production was subsequently optimised in shaken flasks by a central composite design. To this aim, the impact of three crucial variables, namely initial pH and concentrations of ammonium chloride and yeast extract, was investigated. Maximum lipase activity (ca 1230 U dm(-3)) was reached after 188 h of fermentation. The process was subsequently assessed in stirred tank and bubble column reactors of identical working capacity (3 dm(3)), leading to lower lipase production (735 and 430 U dm(-3) respectively) than in shaken flasks. (c) 2006 Society of Chemical Industry.