In this paper, the electrofusion of the ciliated protozoan, Tetrahymena thermophila, is described. Deciliated cells were brought into close membrane contact by dielectrophoresis in a weakly conductive medium. Then cell fusion was induced by application of repeated electric pulses. Up to 20 prestarved, logarithmic or stationary phase cells of the same or different mating types may form a single giant cell. The polykaryons are fully able to regenerate cilia and became motile. After an initial process of partial integration the polykaryons yield viable clones by separation of components. Cytoplasmic exchange between fused components occurs before separation. Cytoplasmically inherited chloramphenicol resistance was transmitted from one strain to another by electrofusion and drug-resistant cybrid strains were generated.