The induction of polyhedroses in the silkworm, Bambyx mari L., was investigated treating the 5th instar larvae just after eodysis with high temperature (hot water bath at 40$^{circ}C$ for 5 minutes or dry heat shock at 40$^{circ}C$ for 30 minutes) and low temperature (5$^{circ}C$ for 24 hours). The results obtained were as follows; 1. Comparing between the frequency of nuclear and cytoplasmic polyhedroses induced by cold and heat treatments (hot water bath at 40$^{circ}C$ for 5 minutes), the induction ratio of the former is clearly less than that of the latter. But if the larvae tested with cold were left at room temperature (25$^{circ}C$) for 30-120 minutes till the next hot water bath (40$^{circ}C$) for 5 minutes and water bath (20$^{circ}C$) for 5 minutes, treatments, the frequency of induced cytoplasmic polyhedrosis was more than that in the case of cold or hot water bath treatment alone. 2. The frequency of nuclear and cytoplasmic polyhedrosis induced by cold and successive heat (dry heat shock at 40$^{circ}C$ for 30 minutes), left at room temperature (25$^{circ}C$) ti11 the second treatment, the frequency of nuclear polyhedrosis was less than that of cytoplasmic polyhedrosis. 3. The reaction of nuclear polyhedra to stains also differs sharply from that of the cytoplasmic type. In a smear of nuclear polyhedra on a slide staining with Giemsa solution remains unstained against a stained back ground, in contrast to this, the cytoplasmic polyhedra take up stain readly.