Four isolates of Saccharomyces cerevisiae were selected in this study for their high ability to produce killer toxin, to investigate possible correlation between their plasmids contents and killer activity. Plasmid’s profiles on agarose gel  revealed presence of two plasmids in each of four isolates.  Yeast cells obtained after plasmid  curing experiments at elevated temperature showed that  cured isolates lost their killing capacity. The plasmid  cured yeast cells originated from three yeast isolates (Scs7, Scvi8 and Scf14 ) showed the possible correlation between plasmid curing and loss of killing ability, whereas isolate Scf4 retained the killing ability in absence of plasmids. Moreover, the extracted plasmid samples were shown  to be sensitive to RNase treatment. These results might suggest that the genetic determinants for toxin production  in three isolates (Scs7, Scvi8 and Scf14 ) of  selected  killer’s yeasts of S. cerevisiae are  encoded by dsRNA plasmid(s) except  one isolate (Scf4), in this isolate  the killer protein  may be  mediated by chromosome.