A selection method for the resistant mutant to black spot disease was developed in Japanese pear by using the host-specific toxin, AK-toxin I. To determine the conditions of selection for the mutants, Chojuro as the reslstant cultivar, Nijisseiki as the susceptible cultivar and γ-1-1 as the mutant with intermediate resistance were used. Leaf disks from the 1 st to 10 th leaves on these shoots were treated with O.O1, O.1 and 1 ppm of the toxin solution. The combination of 4 th leaf with a concentration of O.1 ppm of the toxin solution was adopted as the condition of selection among the combinations of leaf positions and concentrations of the toxin because the mutant with an intermediate resistance could be consistently identified from the original Nijisseiki. The selection method established was as follows : 1) 5 ml of a O.1 ppm toxin solution was placed on two filter papers in 9cm petri dishes. 2) The 4 th leaf from the top on the shoots was marked with a number. 3) Leaf disks were cut from the 4 th leaf, and 25 leaf disks were arranged on the petri dishes in the field. Then, 1 ml of a O.1 ppm toxin solution was added again. 4) The severity of the symptoms was evaluated after keeping the disks at 25°C for 2 days. It was possible for one person to screen about 500 shoots/day by using this selection method. Actually 2 resistant mutants were selected among 2439 shoots that sprouted after cutting back the chronically irradiated trees of the susceptible cv, Nijisseiki with gamma rays. The mutation frequency was 1.3×10^-3 for a dose rate of 12 R/day and 1.6×10^-3 for a dose rate of 18R/day. The induced mutants exhrbited the same type of intermediate resistance to the disease as that observed in the γ-1-1 mutant.