The domesticated silkworm, Bombyx mori has widely differentiated into a various races and strains under the long history of sericulture. After understanding of Mendel low, the discovery and conservation of mutants have also carried out and now more than 420 hereditary traits have been analyzed and mapped on twenty seventh linkage groups (see appendix). In Japan, many institutes (for example University laboratory, National sericaltural station, private company for silkworm seed and prefecture sericaltural station, so on) have been stocked those mutants. They were maintained through world war I and II. Japan lost many things during world war II, but almost of silkworm strains were kept due to the efforts of the staff working at that time. Unfortunately, however, available institutes for the conservation of Silkworm Genetic Resources are decreasing in now due to the decline of sericultural industry of Japan.