Black spot disease, caused by Alternaria alternata Japanese pear pathotype, is the most important and serious disease of the susceptible cultivars of Japanese pear. The fungus produces a host-specific toxin (AK-toxin) which is highly toxic to the susceptible cultivars. Susceptibility to this disease is controlled by a single dominant gene, and all the susceptible cultivars are heterozygous. Mutants resistant to black spot disease were induced by irradiation of susceptible cultivars with gamma-rays. Nine resistant mutants were selected from chronically irradiated ‘Nijisseiki’. One of these mutants was registered as ‘Gold Nijisseiki’. A resistant mutant derived from acutely irradiated dormant scions of ‘Shinsui’ was registered as ‘Kotobuki Shinsui’. One resistant mutant, which was induced from acutely irradiated dormant scions of ‘Osanijisseiki’, displayed unfavorable characteristics. Four resistant mutants were selected from chronically irradiated ‘Osanijisseiki’. One of them was registered as ‘Osa Gold’. A list of these resistant mutants is shown in Table 4. It was confirmed that all of the mutants showed an intermediate resistance to black spot disease and conferred various levels of resistance. Moreover, mutations with a higher level of resistance than that of ‘Gold Nijisseiki’ were induced from ‘Gold Nijisseiki’ by acute and chronic gamma-ray irradiation. The reason why various levels of resistance were induced could not be elucidated.