A chimeric plasmid (pYT760-ADHl) containing the yeast killer toxin-immunity cDNA w a s transformed into a leucine-histidine mutant (AH22) and into four industrial toxin-sensitive yeasts. The chimeric plasmid w a s very stable and expressed toxin production (89.5 -+4.8% killer cells) in two of the transformed yeasts that contained t h e 2 p plasmid, but w a s lost within 10 generations from two other transformed pickle yeasts that did not contain the 2 p plasmid. It suggested that plasmid stability was dependent on the presence of the 2 p plasmid which is naturally present in s o m e yeasts. The plasmid was extremely stable (l00Y0 killer cells) and expressed more toxin in the mutant strain AH22. The effects of dilution rate, D (h-') on plasmid stability and toxin expression were studied in transformed AH22 (AH22m3) and Montrachet 522 (522fl1) wine yeast grown in glucoselimited chemostat cultures. The results show that killer toxin production by AH22iT3 cells increased a s a function of D (h-') and that plasmid stability reached 100% at D 2 0.09 50.01 h-'. However, with Montrachet 522fl1 transformed cells, 100% plasmid stability w a s seen at D 2 0.18 k0.02 h-'. We also challenged the AH22/T3 in chemostat culture (0 = 0.25 h-'1 with an equal number of untransformed cells (AH22). Transformed cells dominated t h e population (100%) within 8-10 h of growth, a time equivalent to two mean residence time. * Paper No 10873 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC 27695. The use of trade names in this publication does not imply endorsement by the North Carolina Agricultural Research Service of the products named, nor criticism of similar ones not mentioned.