The effect of mitochondria on membrane fusion of yeast protoplasts was investigated in order to regulate electrofusion procedures for yeast breeding. We have found that the cell membrane fusion behavior of respiration-deficient yeasts (rho(-)) was remarkably different from that of normal yeasts (rho(+)). Induction of cell membrane fusion in rho(+) protoplasts appeared under the pulse condition (height : 2.5-5.5 kV cm(-1) and duration : 25-430 mu s) after pearl chains were formed. In cell membrane fusants of rho(+) protoplasts, the time interval of morphological change from the long to short slate was 3-11 s. On the other hand, induction of cell membrane fusion in rho(-) protoplasts appeared under the higher pulse condition (height : 4.0-7.0 kV cm(-1) and duration : 20-500 mu s). The time interval of the morphological change from the long to short stale was 110-170 s ill cell membrane fusants of rho(-) protoplasts. The zeta-potential of rho(+) protoplasts was -10-30 mV and that of the rho(-) protoplasts was -25-60 mV. The surface charge of the rho(-) protoplasts was more negative than that of rho(+) protoplasts; therefore, regulation of electrofusion electrofusion among various kinds of yeast strains was possible by changing the surface charge of the protoplasts using mitochondrial mutations.