The No. 908 strain of the silkworm, Bombyx mori, has a dominant gene, Nid-1, in homozygous, which controls the non-susceptibility of the silkworm to densonucleosis virus type 1 (DNV-1). To establish a silkworm race, non-susceptible to DNV-1, the Nid-1 gene was introduced to a practical race, N 150: the F₁ hybrid of N 150 x No. 908 was recurrently backcrossed with N 150 for several generations and individuals having heterozygous Nid-1 (Nid-1/+) were ascertained in every generation as survivors of the oral inoculation of DNV-1. In the 6th generation of backcrossing, Nid-1/+ individuals were sib mated to establish a strain having homozygous Nid-1. In the subsequent generation, the progeny was exposed to DNV-1, and the survivors, whose genotype was supposed to be Nid-1/Nid-1 or Nid-1/+, were further sib mated. Some males which had been used in the sib mating were also mated with female of the susceptible race to test whether the genotype of the male is homozygous Nid-1 or not. In the 2nd generation of the sib mating with males which had been ascertained to have homozygous Nid-1, a progeny line produced all of batches containing only non-susceptible larvae was determined to be the strain with homozygous Nid-1. After improvement in economic characters for several generations, the strain with homozygous Nid-1 was established as the practical race, N 203, non-susceptible to DNV-1. The cross, N 203 x C 150, was authorized by the Minister of Agriculture, Forestry and Fisheries in March 1996, as the commercial silkworm race named “Taisei” for spring rearing.