A new semiselective agar medium, NCP-88, was developed for isolating Clavibacter michiganensis subsp. sepedonicus, the cause of bacterial ring rot, from infected potato plant parts. Important selective components of this nutrient agar, yeast extract, and salts medium are mannitol, poly-myxin B-sulfate, nalidixic acid, and cycloheximide. NCP-88 permitted a better balance between plating efficiency ... [Show full abstract] and selectivity than did other reported semiselective media. In tests of 21 strains of C. m. sepedonicus from pure culture, NCP-88 supported 75133% of the colony numbers that developed on nutrient broth yeast extract agar (NBY). Overall recovery of the pathogen from stem and tuber tissues of symptomatic or asymptomatic potato plants from Idaho and Colorado was significantly greater with NCP-88 than with NBY. Approximately 79% of the total nontarget bacterial population recovered from potato tissues with NBY did not grow on NCP-88 during the 7-day incubation period. Gram-positive nontarget coryneforms on NCP-88 could be distinguished from C. m. sepedonicus by colony morphology and pigmentation. Antagonistic gram-negative bacteria grew on NBY and often prevented the growth of C. m. sepedonicus. These antagonists were sufficiently suppressed on NCP-88 to allow isolation of C. m. sepedonicus. No fungal growth was observed on NCP-88 throughout the 7-day incubation period at 23 C. The new semiselective medium shows promise as an improved tool for detection of viable cells of C. m. sepedonicus in symptomatic and asymptomatic potato plants.