Many transgenic silkworms (Bombyx mori) which produce various silks, e.g. fluorescent silks, have been created. Rearing them in sericultural farms in Japan must be permitted by ministers in charge, according to the Cartagena law, in order to conserve biodiversity. As basic data to evaluate the impact of transgenic silkworm on biodiversity, we investigated behavioral characteristics of silkworms in larval development. In the first three instars, walking distance was the longest on the first day after hatching, or molting under dark condition. On the first day of larval development, the ratio of walking distance to body length was the highest. Comparison of dispersal activity of newly hatched larvae showed that B. mori was inactive, its wild relative B. mandarina was active, and their hybrid was intermediate. After the release of B. mori larvae (newly hatched, forth instar or fifth instar) on mulberry trees in a field, all larvae died before pupation. These results clearly showed that silkworm larvae are inactive and cannot survive under natural conditions.