The present experiment was perform to investigate the frequencies of chromosome aberration with special regard to the chromosome groups and the various time intervals after X-irradiation (60 r) in ormal human foetus cells grown in culture. The cytological preparations were prepared at every 5 through 30 hours after X-irradiation by the air-drying method. 1. The frequencies of chormosome aberration are on the whole decreased as tie elapses after irradiation and this is thought to be due to gradual recovery with time . However, a slight increase in frequencies is observed at 25 and 30 hours after irradiation respectively. This shows that the cells at the these periods are more sensitivity to X-irradation , and those cells are thought to be at G$_2$ and late S stage at the time of irradiation respectively, so t is evident that G$_2$ and late S stages a the time of irradiation respectively , so it is evident that G$_2$ and late S stages are more sensitive to X-irradiation than any other stages. 2. The frequencies of chormosome aberration are decreased in descending order of chormosome group number. The differences among these frequencies are highly significant statistically . Therefore it can be concluded that there is a highly significant difference in radiosensitivity among chromosome groups. that is, the chromosomes of the group A are the most radiosensitive , followed by B, C, D ,E and G in descending order.