The total number of viable bacteria found in Solanum tuberosum stems and tubers was found to vary from less than 1 x 10³ to 4.7 x 10⁷ cells per cm³ in stems and from 0 to 1.6 x 10⁴ cells per cm³ in tubers. About 75% of both stems and tubers had populations at the lower end of the range but there was no significant correlation between bacterial counts in stems and tubers of the same plant. Bacteria found in potato tubers included species of Micrococcus, Pseudomonas, Bacillus, Flavobacterium, Agrobacterium, and Xanthomonas. Also present were coryneforms and some others which were not identified to genus but were gram negative. Some of the coryneforms were morphologically indistinguishable from the bacterial ring rot organism (Corynebacterium sepedonicum) but non-pathogenic and biochemically different from it. About 5% of stem smears of healthy plants showed more than 30 gram positive rods per microscope field. All the Bacillus spp., one Pseudomonas sp. and three unidentified species were found to inhibit C. sepedonicum in vitro. An antibiotic was partially purified from the Bacillus sp. showing the greatest amount of inhibition. This species was also antagonistic toward the following potato pathogens: Pseudomonas solanacearum, Erwinia atroseptica, E. carotovora, Alternaria solani, and Phytophthora infestans. Physical and chemical tests showed that both a lipid and a peptide antibiotic were involved in the antagonistic effect. / Land and Food Systems, Faculty of / Graduate