Population size of Clavibacter michiganensis subsp. sepedonicus in potato cv Russet Burbank and plant response as affected by drought were assessed in a greenhouse experiment. Water deficit stress and no stress treatments, and inoculum densities of 0 or 2 X 10��� cfu C. m. sepedonicus/seed piece were arranged factorially. Stem populations of C. m. sepedonicus were significantly lower in the water deficit stress treatment compared to the non-stressed treatment at every sampling date in both experiments. In seven of the eight harvests the number of C. m. sepedonicus cells/g of stem tissue for the water deficit stress treated, infected plants was a factor of 10 lower than the non-stressed treatment. Foliar symptoms of bacterial ring rot were not observed, but symptoms developed in tubers. Compared to the noninoculated control inoculum reduced aerial biomass from 12 to 21% and tuber yield from 15 to 38% in samples taken four times after the drought was terminated. Reduction of these same variables due to water deficit stress ranged from 17 to 21% and 15 to 41%, respectively, compared to the non-stressed control. Therefore, both water deficit stress and C. m. sepedonicus had similar effects on growth and tuber yield of potato. / Graduation date: 1995