The base temperature and the thermal time of the evening primrose crop (Oenothera spp.) were determined by a seven different temperature regimens experiment. The duration (days) and the average air temperature of each development phase and the whole plant cycle were measured for each temperature regime. The base temperature and the thermal time were defined by the linear regression method between the rate of development (1/days) and the average air temperature of each development phase. The regression coefficients represented the base temperature (Tb = -a/b) and the thermal time (Ө = 1/b). The base temperature of the germination, rosette, stem elongation, flowering formation and fruit maturation phases were 0.1, 2.2, 1.8, 5.7 and 6.8°C, respectively. The thermal times for the respective phases were 130, 464, 304, 273 and 297 degree-days. The base temperature of the whole cycle was 1.1°C and the thermal time was 2624 degree-days. Because the rate of development and the average air temperature of the flowering phase had a positive relationship its base temperature and the thermal time were not determined. The number of flowers was positively related to the air temperature. The base temperature and the thermal time of the flowering and fruit maturation phases together were 0.1°C and 1369 degree-days, respectively. The low base temperature of the germination, rosette and stem elongation phases and their relative high thermal time, principally the rosette phase, allow their duration to be long enough during the cool months. Then, the flowering formation and flowering phases will occur during the warmest months of the year, even if the sowing is made in the winter. The flowering phase will stop as soon as the temperatures start to fall. The duration of the last phase will be long enough to allow the most seeds to be matured and released by the capsules.