Production of recombinant Aspin, a aspartyl protease inhibitor homologue produced by the parasitic nematode Trichostrongylus colubriformis, in Escherichia coli is reported. Culture conditions were investigated for maximizing the production of Aspin in soluble bioactive form as opposed to inclusion body. High growth and expression rates caused preferential production of inclusion bodies. In fed-batch fermentations, controlling expression at low values by decreasing the bioreactor temperature, dissolved oxygen level and concentration of nutrients, all proved effective in enhancing the production of soluble Aspin. A high volumetric titre of 220 mg/L Aspin was attained in batch fermentations induced with 2 g/L l-arabinose with the postinduction temperature reduced to 25 • C from 37 • C. The pH and dissolved oxygen levels were not controlled, as acidic final pH values and low dissolved oxygen levels favored production of soluble Aspin.