Yeast killer toxins were identified as proteins which were named killer factors or killer toxins and the producing strains were termed killer yeast. Killer yeasts secrete proteinaceous killer toxins lethal to susceptible yeast strains. The killer strains are insensitive to their own toxins.We aimed at this work to Screen for the killer and sensitive strains among different yeast strains and to study the effect of the environment on this killer character and the different curing treatments for detecting the genetic determinants responsible for the killer toxins. Thirty two yeast strains were used for cross-reaction assay to select some killer strains against 8 yeast strains. Five of these strains were used to study the effect of the selected killer yeasts on their growth. The killer strains obtained were characterized in relation to the optimal temperature and pH for activity. Most killing yeast strain was Candida glabrata .The killed yeasts represented all different genera under study except for S.Cervisciae ATCC 6037. Curing experiments showed that dsDNA plasmid are responsible for the killer character in these killer strains except for the killer strain S.Cervisciae.While all these strains except for C. krusie CMGB94 lost their killing activity with elevated temperature curing. All strains showed complete curing and lost their killing activity at both PH3 and PH8, while showed optimum killing activity at PH4 and 5, while partial curing occurred at PH 6 and 7.Also all strains except for C. krusie CMGB94 showed dsrRNA as genetic element responsible for the killer character.Yeast killer toxins are thought to serve the purpose of competitor killing and thereby provide a selective advantage to the producing species. Killer yeasts and their toxins have many potential applications in environmental, medical and industrial biotechnology.