Triops granarius (Lucas) (Notostraca: Triopsidae) lives in paddy fields from the Kanto district to northern Kyushu, Japan. Changes in the size distribution of this species were examined in the paddy fields and then the effect of light on hatching was examined under quasi-natural and laboratory conditions. Adult tadpole shrimps were found about one week after irrigation and plowing in two paddy fields in Sakai, Japan. They developed rapidly and disappeared altogether about one month later. Under conditions of natural daylength and temperature, eggs laid in the soil did not hatch without being removed from the soil. Under constant light at 25°C, the lower the light intensity was, the longer the eggs took to hatch. Moreover, most eggs kept in constant darkness did not hatch, but many of them hatched within a short period after being transferred to constant light with an intensity of 0.3 W/m2 or more. Because a 1-h light pulse was found to induce hatching, light is considered necessary for the resumption of embryonic development. These results suggest that eggs of T. granarius laid in the soil do not hatch without exposure to light; consequently, this species has a univoltine life cycle in the paddy fields. Histological observations revealed that under constant darkness, embryonic development was arrested at an early stage of organogenesis, in which the nauplius eye had not yet formed. We discuss the role of light in the regulation of embryogenesis in T. granarius.