A mutagenicity testing system using a specific locus mutation of Bombyx mori was introduced from National Institute of Genetics in Japan. In the system, mutagenicity could be detected by the egg color manifested by the pe and/or re genes, which is a kind of recessive visible mutation of the insect. The efficiency to detect mutagenicity of the system was examined and improved. To promote the sensitivity of the system to mutagens, eight varieties of Bombyx mori were tested for their sensitivity to two mutagens, EMS and MMC. Two varieties of the silkworm, N12 and C5, were finally selected as the most sensitive ones. The most sensitive development stage of the silkworm to mutagens was mid and late pupal stages for female and male, respectively. The system will be applied to test unknown mutagens after some more detail examination about its sensitivity.