The lifespan of the imago in the silkworm, Bombyx mori, was considered to be around one week. We have investigated the lifespan in imago and the distributions of the number of dead individuals per day using 22 silkworm strains maintained in Tokyo University of Agriculture and Technology to examine any hereditary factors. The average lifespan in imago was 2.14-12.42days, and significant differences among strains were noted. There were lifespan differences among local varieties; average lifespan in imago of Japanese strains were slightly shorter than European and Chinese strains. Histograms of death frequency were classified into the three peak patterns of one clear peak, no clear peaks and one sharp peak in short lifespan in imago. In all five investigated Japanese strains, the graphs showed one clear peak. These results suggested the intervention of some genes in the lifespan.