Several E. coli-C. acetobutylicum shuttle vectors were constructed and used to transform two C. acetobutylicum strains ATCC 824 and NCIMB 8052. Other than pSYL2, none of these vectors were able to transform ATCC 824 due to the presence of a restriction system. However, all of them could transform NCIMB 8052 with efficiencies of 8 x 10(2) - 6 x 10(3) transformants per mu-g DNA.