Kluyveromyces marxianus can be transformed with an efficiency of 10(5) transformants/microgram of DNA by a replicative plasmid using electroporation. In order to obtain this efficiency, we isolated ura- mutants cells which can be complemented by the URA3 gene from Saccharomyces cerevisiae. The URA3 gene and KARS2, a replicative origin from Kluyveromyces lactis which functions in K. marxianus, were ligated together in a plasmid which can be used as a vector to transform this strain.