Seven isolates of Puccinia hoyiana tolerant to oxycarboxin (5, 6-dihydro-2-methyl 1, 4-oxathiin-3-carboxanilide-4, 4-dioxide) were detected on pot chrysanthemums in Nara and Mie Prefectures of Japan.Tolerant isolates required oxycarboxin concentration 5-10 times greater than sensitive isolates to inhibit teleutospore germination, and 10-20 times greater to inhibit basidiospore formation. Plants inoculated with tolerant isolates showed more severe disease symptoms than plants inoculated with sensitive isolates.Since amobam, captan, maneb, triforine and wettable sulphur were effective against oxycarboxin-tolerant isolates, these fungicides may be included in a spray program. Oxycarboxin-tolerant isolates, however, showed cross-tolerance to carboxin (5, 6-dihy-dro-2-methyl-1, 4-oxathiin-3-carboxanilide).