The squalene-hopene cyclase from Bacillus acidocaldarius cytoplasmic membrane, was purified to homogeneity by solubilization with Triton X-100, chromatography on DEAE-cellulose, phenyl Sepharose and two gel-filtration columns. The enzyme monomer had a molecular mass of 75 kDa. The sequence of the first 23 amino acids was determined by Edman degradation. The enzyme activity was efficiently inhibited by n-alkyldimethylammonium halides with alkyl chain lengths between 12 and 18 C atoms. Inhibition was also observed with (5-hydroxycarvacryl)trimethylammonium chloride 1-piperidine carboxylate, dodecyldimethylamine N-oxide, azasqualene and farnesol. Competitive inhibition with dodecyltrimethylammonium bromide, (5-hydroxycarvacryl)trimethylammonium chloride 1-piperidine carboxylate and dodecyldimethylamine N-oxide was demonstrated by Lineweaver-Burk plots.