Based on individuals collected from four populations in Japan, the ploidy of Ixeris chinensis subsp. strigosa was investigated to know how triploids and tetraploids occur. Except in one small population where only two individuals were collected, the populations contained both triploids and tetraploids with the frequencies of triploids in each population varying from 38% to 60%. Their consistent coexistence suggests that the triploids and tetraploids had migrated together and not independently from continental Asia. Compared to the past time when more populations existed, populations of I. chinensis subsp. strigosa are now very few in Japan and appear to be seriously threatened by extinction.