The RNA content of fertilized egg of Bombyx mori was shown a continuing great increase reaching peak at the 5th day after egg laying and a slight decrease there after. Such a change of RNA content is considered to be directly associated with the formation and development of egg embryo of silk worm. (2) The RNA content of nonfertilized egg is much less than that of fertilized one at first day of egg laying and it increased slightly until 4 th day after egg laying then decreased. (3) The RNA content of fertilized egg irradiated by gamma-ray (3,000 r) was shown a slight increase until 2nd day after irradiation, but no change was observed there after. This fact shows that irradiation suppressed the biosynthesis of RNA silk worm egg. (4) The RNA content of HCl treated silk worm egg was shown a continuing steep rise until 7 th day after the acid treat, while no change was observed in the non-treated egg. The RNA content of HCl treated egg with irradiation of gamma-ray (1,500 r), decreased until 3 rd day after irradiation in contrast to that of non-irradiated group, but it increased rapidly from 4 th day until 7 th day after acid treating.