This paper presents the results of experiments on the relative biological effectiveness (RBE) of 14 MeV fast neutrons as compared with 137Cs γ-rays for recessive visible mutation induction on the primary meiotic spermatocytes in the silkworm. 1-day-old fifth instar silkworm larvae were treated with five different doses (167, 250, 500, 684 and 1000 R) of γ-rays, or three different doses (160, 250 and 540 rad) of 14 MeV neutrons. Mutations at the pe and re loci were detected by the egg-colour specific locus method. Primary meiotic spermatocytes showed an extremely high radiosensitivity of cell killing which has not been previously found in postmeiotic cells. With 1000 R of γ-rays and 540 rad of neutrons, more than 90% of the cells in meiotic divisions were killed. However, these effects were negligible with lower doses either for neutrons (160 and 250 rad) or γ-rays (167, 250 and 500 R). Hence the results presented are from the remaining low doses for both radiations. The average mutation rate obtained on the two loci was 0.9·10 −6 per rad for γ-rays and 1.9·10 −6 per rad for neutrons. The mutation rate for γ-rays was somewhat lower than that of spermatids which have the highest rate throughout spermatogenesis. The RBE of 14 MeV neutrons was determined only for whole-body mutants because the frequency of fractional-body mutants was nearly the same as the control. The initial dose mutation frequency for both loci and both radiations were almost linear at lower doses, though at higher doses they tended to reach a maximum followed by a decline in mutation frequency. The RBE was calculated on the assumption that the dose-response curve was linear in lower dose regions. Thus, RBE values, as the ratio of the slope of the weighted linear regression line for neutrons to γ-rays, were estimated to be 1.7 at the pe locus and 2.2 at the re locus. These values are about one-third of those for mature sperm and are much the same as those in primordial germ cells and in spermatogonia.