Summary(#br)A method was developed for the introduction of plasmids into Clostridium botulinum by electroporation. A 4.4 kb plasmid vector, pGK12, which contains genes for resistance to erythromycin (Em r ) and chloramphenicol (Cm r ) was electroporated into C. botulinum type A (Hall A). The highest transformation efficiency was obtained using midlog phase cells, 10% PEG 8000 as the electroporation solution, and 2.5 kV field strength. The transformation efficiency was highest (?10 3 transformants/μg of DNA) when 1 μg of plasmid DNA and 4 × 10 8 CFU/ml of recipient cells were used. Plasmid DNA recovered from the transformants was indistinguishable from that introduced on the basis of restriction enzyme digestion and agarose gel electrophoresis.