Chromosome number variation within and between individual plants and mitotic abnormalities in root-tip cells were observed in the Amagahara family (2n=56) of garden chrysanthemum and Ch. shimotomaii (2n=54). Abnormal mitotic divisions at anaphases and telophases were found in both the Amagahara family (2.12%) and Ch. shimotomaii (1.05%). The highest rate of mitotic abnormalities observed was found in a plant of "Ki-amagahara," which was placed in a high temperature condition in the daytime, at a frequency of 5.5 per cent. It is probable, therefore, that those mitotic abnormalities are common in hexaploid chrysanthemums and the frequency of abnormalities is affected by the environmental factors, especially temperature. The chromosome numbers of the root-tip cells of the Amagahara family counted varied between 2n=52-58. And also in the cells of Ch. shimotomaii 2n=49-56 chromosomes were counted. The greatest range within a single root was found in "Ki-amagahara" (2n=54-58). Accompanying chromosome variation within a single root there was also variation between roots of a single plant in both the Amagahara family and Ch. shimotomaii. In the five plants with different chromosome numbers in the variety "Amagahara" (2n=56), four plants with 54 and 55 chromosomes showed no visible phenotypic changes, while the other with 52 chromosomes was distinguished by its weakness and smaller leaves. One sport of the variety "Ki-amagahara" which accompanied vigorous growth and higher plant heights had 2n=57 chromosomes instead of 2n=56. Based on the results obtained, the relationship between sporting process and chromosome variation are discussed.