Screening of fatty acid fed bacteria which are not natural butanol producers identified increased membrane cyclopropane fatty acid as providing improved butanol tolerance. Increasing expression of cyclopropane fatty acid synthase in the presence of the enzyme substrate that is either endogenous to the cell or fed to the cell, increased butanol tolerance. Bacterial strains with increased cyclopropane fatty acid synthase and having a butanol biosynthetic pathway are useful for production of butanol.