Bombyx mori silk gland DNA was compared by several techniques with DNA from larval carcass tissue and pupal testes in order to determine whether the enormous DNA increase in these cells is a uniform replication of the entire B. mori genome (polyploidization). The DNAs had the same buoyant density in CsCl gradients with no evidence of satellite bands. Molecular hybridization assays showed that the genes for ribosomal RNA and transfer RNA are replicated in exact proportion with the bulk of the DNA during the DNA increase. Also the total DNA nucleotide sequences of the different tissues were indistinguishable by qualitative and quantitative analyses. The repetitive sequences were compared by the relative ability of complementary RNA synthesized from one DNA to hybridize with another. The abundance of nucleotide sequences with different kinetic complexities was compared by DNA reassociation studies and found to be indistinguishable. Previous studies (Suzuki et al., 1972) had shown that the developing silk gland cell does not amplify the genes which would facilitate fibroin synthesis. These studies demonstrate conclusively that polyploidization of the silk gland involves replication of all its genes to at least 4 × 10 5 times their concentration in a haploid sperm nucleus, probably by seventeen or more rounds of complete genome replication without cell division during larval development. † Present address.