The silkworm, Bombyx mori, at various larval stages was exposed to high temperature (36℃) for 48 h, and the effects on the survival rate and cocoon characters were investigated. The heat treatment in larvae in each instar caused a reduction in the survival rate, and the heat susceptibility increased with age. Within the same instar (from 1st to 4th instars) heat treatment of the larvae at early or later stages resulted in a much higher survival rate than that of larvae treated at the middle stage. Results of heat experiment conducted at various times during the 5th instar showed that larvae treated in the early half of the middle period and late stage before mounting were more susceptible to heat which reduced their survival rates compared with the larvae treated in the other periods. Heat treatment applied at various times in larvae also reduced the cocoon weight and cocoon shell weight, and the period of heat susceptibility was almost similar to that related to the reduction of the survival rate. When the cocoon weight and cocoon shell weight were reduced markedly by the heat treatment, the cocoon shell ratio tended to show a high value. Heat treatment applied to the larvae in each instar (from 1st to 5th instars) prolonged or shortened the duration of the feeding period by less than one day. Within the same instar, heat treatment at the early stage prolonged the duration of the feeding period, whereas the treatment at later stages shortened the feeding period. The results of the present study indicated that the heat treatment affected the survival rate and cocoon characters depending on the larval stage at which the treatment was applied, and that the 5th instar larvae at the early stage were most susceptible to the heat treatment. Accordingly, for testing the heat resistance of the silkworm, it is recommended to expose 5th instar larvae at the early stage to high temperature (36℃) for 48 h.