Variations in egg-number deposited per female, sex-ratio, duration of growth period of insects, survival-rate during growth period, body weight and heat-tolerance of adults have been observed under environmental conditions both constant and variable. Experimental materials were Azuki-bean weevils, Callosobruchus chinensis. (1) When the environment under which insects develop is constant, some of the above described say physiological characters are variable according to different generations. That is to say, heat-tolerance considerably increases in the third and sixth generation. Egg-number and survival-rate show temporary decrease in the second and third generation, respectively. Body weight, sex-ratio and duration of growth period, however, are constant in all generations. Variation-coefficient in heat-tolrance decreases with the increase in heat-tolerance (median lethal time). That in egg-number deposited and survival-rate increase with the decrease in both characters. (2) Considering the mean and variation-coefficient of insect characters, it may be assumed that the cause of the changes in the characters when the environment is constant is not to be attributed to the changes in environmental temperature and humidity, but rather to the changes in the physiological conditions of the insect itself: (a) The increase in heat-tolerance is assumed to be due to the fact that by the extinction of individuals of weaker viavility and tolerance during their growth periods, the tolerance of insect population becomes homogeneous and high. (b) The decrease in egg-number and survival-rate is presumed to be due to the fact that the fecundity and viavility of certain individuals in insect population decrease on account of the harmful effect of inbreeding, which causes the fecundity and viavility of the entire population to become heterogeneous and low. (3) When the environmental conditions are variable by the different seasons, the variations in egg-number, duration of growth period, survival-rate during growth period and body weight of adults seem to depend upon the changes in environmental temperature. Strictly speaking, however, the variation in the characters does not depend upon the environmental temperature alone, but the characters such as sex-ratio, survival-rate and heat-tolerance are modified by some physiological changes in the insect itself or the after effect of environmental factor in parent generation. (4) Variation-coefficient of the characters does not so frequently change as the mean value, when the environmental conditions of the insect vary. However, those of two characters, i. e. egg-number deposited and heat-tolerance are variable by the different seasons. The increase in variation-coefficient of egg-number may be due to the fact that the insect population becomes heterogeneous in regard to its fecundity, whearas the decrease in that of heat-tolerance is presumed to be attributed to the fact that the population becomes homogeneous in regard to tolerance of the insect to temperature by the environmental temperature.