An electro-transformation system has been developed for Nocardia asteroides and Nocardia corallina by using a Nocardia-Escherichia coli shuttle vector. The shuttle vector, named pCY104, was constructed by joining a 2.5-kb cryptic N. asteroides plasmid pCY101 with the E. coli plasmid pIJ4625. The resistance genes for kanamycin, chloramphenicol, and thiostrepton on plasmid pCY104 were expressed in N. asteroides and N. corallina. The transformation method was optimized for N. asteroides, and transformation efficiency of 8 x 10(4) transformants per mu g plasmid DNA was achieved routinely.