Presently, cytomorphological investigations have been carried out in five populations of Aster thomsonii from different altitudinal areas of district Sirmaur (H.P.). The investigations revealed three cytomorphotypes: diploid (2n=18), tetraploid (2n=36) and hexaploid (2n=54). The chromosome count of 2n=54 (hexaploid cytotype) has been reported for the first time at world level. All the populations have anomalous meiotic course and reduced reproductive potential of 72, 81 and 59% for diploid, tetraploid and hexaploid, respectively.