Abstract: Based on a karyotypic analysis of 13 populatlons，the karyotypic differentation in Allium Macrostemon Bunge was investigated in this paper．The twelve populations from China were all found to be tetraploids with the chromosome number of 2n＝32，while the re maining one from Japan was a pentaploid with 2n＝40．In the asymmetry，all the karyotypes belonged to Sebbins’2A type. Obvious karyotypic differentiation among the populations， which was highlighted by chromosomal aberrations，including mainly pericentric inversion and the Robertsonian Or unequal reciprocal translocations，was detected．Hence the kary otype of this species was somewhat polymorphic．The karyotypes of the tetraploid popula tions fell into two types．One was formulated as 2n＝4x＝32＝26m(2sat)+6sm(2sat)．The populations from Huashan，Shangnan，Changchun，and Dujiangyan had this type，though in the population from Dujiangyan a pair of median satellited-chromosomes was found to have changed into subterminal ones．The other was formulated as 2n＝4x＝32＝24m(2sat)+8sm (2sat)．The populations from Fengyukou，Dafeishui，Wenchuan，Wuhan，Honghe，Baoxing，Hangzhou and Xiangshan all had this type，but in the three populations from Baoxing， Hangzhou and Xiangshan，a pair of chromosomes had undergone structural aberration．Basi cally the karyotype of the pentaploid population from Japan fell into the first type，though in it a pair of median chromosomes had been replaced by a pair of submedian ones. Factors which may possibly result in the karyotypic polymorphism，including those of environmental conditions，breeding system and polyploidy，were analyzed．We believe that the diversified habitats might have induced the chromosomal structural aberratoins，and the facultative apomictic reproduction and polyploidy might help to surmount the obstacle of reduced fertili ty and thus maintain chromosomal aberrations in populations．An analysis of the modem distribution pattern shows that Japan may be the differentiation center of Allium macroste-mon．

Key words: Allium, A. macrostemon, Karyotype, Intraspecific differentiation