Protocol describes a method for transformation of the Tetrahymena using electroporation. The vector is electroporated into cells after mating, where it is incorporated into the DNA of developing macronuclei. Because T. thermophila can be propagated indefinitely without conjugation, transformation of the macronucleus provides a way to obtain stable somatic transformants. DNA vectors transformed using this protocol include those containing drug-resistant versions of Tetrahymena genes.