The biology of Unaspis yanonensis (Kuwana), a major economic pest on citrus in Japan, was studied at Hagi-city, western part of Honshu, Japan in 1959 and 1960. This coccid has three generations a year at Hagi-city. Each generation is represented by two peaks of the population-curve. The period of occurrence, the duration of each generation and its abundance seem to be highly affected by the solar energy and temperature. Some scales of the third generation stop oviposition and overwinter whenever the temperature becomes low, but again begin to oviposit eggs in the next spring. The first instar covers 21.8 days, the second 22.1 days and the third 34.2 days in the first generation (total average covers 77.7 days), but each instar of the second and third generations (except the first instar of the second generation) covers much longer period. During the course of development the mortality of those living on the part of crown of citrus or on the part of middle hight is high, while those found on the lower part live longer making the percentage of damage highest. The scales prefer the shady environment with high temperature. Therefore, the management of the citrus groves should be made so as to avoid such a favorable environment against the scales. The scales on the fruits, stems or leaves which are dropped on the ground may become the source of the propagation of this scale insect.