The effects of acute irradiation on the kinetic indices of primary spermatogonia of the silkworm have been studied in an attempt to gain cytological insight into the mechanisms of dose-fractionation effects on the induction of mutation. 1. ( 1) Kinetic studies, with the use of colchicine treatment and the [ 3H]thymidine-labeling technique, on primary spermatogonial populations in unirradiated larvae showed that primary spermatogonia of this insect had a mean generation time of 34 h, of which the relative time occupied by the various phases of cell cycle was roughly as follows: G 1, ∽ 40%; S, ∽ 40%; G 2, ∽ 18%; and M, ∽ 2%. 2. ( 2) Cytokinetic studies on the effect of acute irradiation (1000 R) on the population of primary spermatogonia showed that during the first 24 h after irradiation the relative frequency of cells in different phases of the cell cycle changed noticeably. Thus, at 24 h as many as 58% of the total population was shown to be in the G 2 population in contrast to 18% of the unirradiated primary spermatogonial population. Furthermore, only 3%, instead of 40%, in the control, was in the S phase at this time. 3. ( 3) It is tentatively concluded that enhanced mutation frequency in dose-fractionation experiments was primarily due to an increase in the relative frequency of cells in the G 2 population. However, the possibility remains that accumulation of cells in late G 1 might also be responsible for the enhancement of mutation frequency by dose fractionations.