Two different killer phenotypes were detected among K+ (killer) yeasts isolated from spontaneous wine fermentations using a plate bioassay. The two phenotypes differed in their degree of killer activity, and were designated as SK+(strong killer) and WK+(weak killer). Strains showing either phenotype were assayed for expression of killer activity under different growth conditions. Growth in must negatively affected expression of the killer activity of both phenotypes. The supernatant fluids from must cultures showed a lower killing effect than those from yeast phosphate dextrose broth (YPDB) cultures. The ability of the two K+ phenotypes to prevail on K-sensitive yeasts was studied in mixed-culture fermentation experiments. Under these conditions, only strains showing SK+ phenotype were able to prevail on the K-sensitive yeasts. These results suggest that the K+ phenotype could play a relevant role in spontaneous fermentations provided that the strain exhibits an SK+ phenotype, and that the latter phenotype should be preferred when selected K + strains are to be used as fermentation starters.