Plasmids derived from pNG2 or RSF1010 were introduced into strains of Arcanobacterium (Actinomyces) pyogenes by electroporation. Electroporation conditions were varied systematically to give a maximum electroporation frequency of 3.7 x 10(5) CFU/microgram DNA at 1.5 kV/cm and 246 omega, resulting in a time constant of approximately 10 ms. The A. pyogenes transformants expressed plasmid-encoded resistance to chloramphenicol, erythromycin, kanamycin, and streptomycin. The source of incoming DNA affected the growth rate of transformants, but not the electroporation efficiency. This is the first report of genetic transformation of the veterinary pathogen A. pyogenes.