Nephotettix cincticeps UHLER and N. apicalis MOTSCHULSKY were compared in some morphological and physiological characters and the following results were obtained.<br>1. In the incubation period of egg, N. cincticeps shows equations, Y=0.0115X-0.165 and y(X-14.3)=87.0 (Y: developmental velocity, y: developmental period, X: temperature), N. apicalis shows Y=0.0102X-0.146 and y(X-14.3)=98.0. These equations mean that N. cincticeps has a higher developmental velocity than N. apicalis, developmental zero point being the same and thermal constant being smaller in the former. The difference is thus in small degree but it persists in the whole range of temperature used.<br>2. In the larval development too, N. cincticeps has higher developmental velocities in both sexes, though a little irregularity is found at 32.4°C, that is, the velocities deviate from the regression lines downwards in both species and sexes. Regression equations are as follows: N. cincticeps male, Y=0.00496X-0.0664 and y(X-13.4)=201.4; female, Y=0.00456X-0.0601 and y(X-13.2)=219.4; N. apicalis male, Y=0.00480X-0.0647 and y(X-13.5)=208.3; female, Y=0.00440X-0.0582 and y(X-13.2)=227.3. But the differences in developmental velocity are in such a degree that the difference between the two sexes of the same species exceeds that of the two species of the same sexes.<br>3. In the pre-ovipositional period, no clear difference is found at higher temperature, but the elongation with a fall of temperature is higher in N. apicalis than in N. cincticeps.<br>4. In the daily rate of oviposition during the first 7 to 10 days after the beginning of oviposition too, no clear difference is found. At 32.4°C the daily rate of oviposition is clearly depressed in both species, probably owing to the harmful effect of high temperature.<br>5. The body size of adult decreases clearly with a rise of temperature in N. apicalis. On the other hand, in N. cincticeps a slight increase is found in the female, and in the male it decreases but in a lesser degree than in N. apicalis.<br>6. Sensitivity to photoperiod was studied by the elongation of the larval period and the reduction of the body size of adult resulted under a short day photoperiod of 8hr. As already reported, N. cincticeps shows a diapause in the 4th and/or the 5th larval stage under a short day photoperiod of 8hr at 20°C and adults of the spring form are resulted. In N. apicalis merely residual elongation was observed, accompanying a little reduction of the body size of adult at 20°C and a short day photoperiod of 8hr. The rate of reduction is in a lesser degree than in N. cincticeps.<br>7. The relation between the two closely allied species which have dissimilar geographical distribution is discussed and N. cincticeps is characterized as a species having a northern distribution area by its higher sensitivity to the short day photoperiod compared with N. apicalis, a southern species.