We have identified karyotypic mutants of Magnaporthe grisea isolate Inal68. The mutants have a karyotype with an extra band of chromosome-ν in addition to six bands of the normal karyotype in contour-clamped homogeneous electric field (CHEF) electrophoresis. We named a karyotype of the original isolate as α′, and that of mutants as α′. The mutation has not affected the DNA fingerprint pattern, host specific pathogenicity (pathogenic race), or crossing abilities with the isolate Guy11. Karyotypic analyses of isolates from a repetitive single-spore isolation indicated that the frequency of mutation was 12.5% and that the mutation was limited in the manner of a to α′, and never reverted. Hybridization analyses showed that chromosome-ν originated from the chromosome in band IIIb of type α and might be produced by deletion.