Present report was concerned with analysis on the relation between the formation of fibroin and sericine in the silkworm and the rearing temperature.<br>1. With analysis on the effect of the rearing temperature on the formation of fibroin, the rate of formation of this protein was larger under the higher temperature (Fig. 1A). This fact was different from that the growing rate of the silkglands at 30°C was approximately the same as 25°C. The peak of the curve appeared at Δ4 in the accelerated formation of fibroin independently of the rearing temperature (Fig. 1B). The effect of the rearing temperature on the formation of sericine showed that the activity of the formation at 30°C was inferior to 25°C (Fig. 2A) and the peak of the accelerated formation curve appeared at Δ4 independently of the temperature (Fig. 2B). The ratio of the amount of sericine to total silk protein in the silkglands fell with the progress of larval growth. This fact showed that the formation of fibroin was delayed to that of sericine. Moreover, change of the values of those ratios at 30°C was faster than 25°C and 20°C and the both temperatures showd the same values (Fig. 3). However, the values finally constricted was the same in every temperatures and the ratios of scouring loss of the cocoons were 27.5% at 30°C, 27.2% at 25°C and 27.8% at 20°C.<br>2. In the relative variation of the amount offibroin and of sericine to the body weight, three lines were fundamentally drawn (Fig. 4A and B) and the quotients of the formation of fibroin were 2.00, 1.31, 2.20 at 30°C, 3.27, 0.82, 2.00 at 25°C and 2.80, 1.03, 2.00 at 20°C, in respective lines. Those of sericine were 1.87, 0.97, 1.16 at 30°C, 4.23, 0.78, 1.71 at 26°C and 4.25, 0.47, 1.64 at 20°C.<br>3. The ratio of fibroin to total protein in the middle division of silkgland was larger under the higher temperature than the lower temperature (Fig. 5).<br>4. The variation of the quatitative ratio of fibroin to the posterior division of silkgland showed that the ratio rapidly rose at 30°C than the other temperatures (Fig. 6). As shown in the growth curve of the tissue of the posterior division of silkgland, the growth nearly ceased and the value of the tissue in the maximum growth was markedly lower at 30°C than the other temperatures.<br>5. The rate of the sum of the tissue weight of the posterior division of silkgland during the three days, in which the formation of fibroin was more active, coincided with that of the weight of cocoon layer between three rearing temperatures (Table 1).<br>6. The synthetic activity of fibroin, i.e., the amount of the enzyme system of fibroin synthesis was lower under the higher rearing temperature than the lower temperature.