Nine different strains of Streptomyces ('S. scabies') have been isolated from scab lesions on potato tubers. These strains were obtained from different cultivars grown in different geographical areas. Four different types of inhibitory reactions were observed when these strains were tested in pairs with each other. Three of these inhibitory reactions resembled the lethal zygosis phenotype that has been reported for other species of Streptomyces. All of the strains exhibited at least two of these inhibitory reactions and were sensitive to the inhibitory reactions of at least six of the other isolates. Five of these isolates were shown to harbour plasmid DNA.