Summary In order to investigate the difference of changeability in chromosome number and structure between two pairs of homologous chromosomes in Haplopappus gracilis callus, karyotype analyses of a number of metaphase chromosome spreads were carried out. More than 90% of cells examined showed chromosome number, 2n=4 as same as normal controls, however the cells with the same karyotype as normal controls decreased to 52.8%, in total. Change rate of centromeric position increased as culture period became long from early to late. Centromeric transposition rate became high in the callus of low growth rate. Change rate of centromeric position of the 2nd chromosomes which have satellites was about 2.5 to 3 times higher than that of the 1st chromosomes in all the concentrations and periods. The 2nd chromosomes of normal controls are subterminal chromosome type, but the majority of changed 2nd chromosome in callus cells was submedian chromosome type. This may probably due to breakage of the long arm of the 2nd chromosome and suggests the existence of breakpoint(s) in the long arm of the 2nd chromosome.