A tadpole shrimp, Triops granarius appears in paddy fields a few days after the water submergence and ploughing. The Triops species have many similarities in their ecological characteristics with those of the annual plant weeds in paddy field, such as the life cycle in an annually disturbed habitat, the wide range of food or nutritional requirement, the high persistency of eggs or seeds in drought, the rapid development and maturation, the great overlapping of growing and reproductive stages, the high reproductive power, and the self compatibility in reproduction. As some annual weed species in paddy fields are light-sensitive in their seed germination, T. granarius eggs were also examined in this respect and it was found that the light condition has significant effect on their egg hatching.The eggs, collected from a paddy field with soils at Ibaraki City, Osaka (Experiments 1 and 2), and oviposited in the laboratory (Experiment 3), were submerged in water and the rate of egg hatching was examined in the darkness and under the various day lengths of illumination by the fluorescent lamp (Toshiba white 20W NL, 40 cm apart) at 20°C. A high percentage of eggs hatched rapidly under the light condition between 8 and 24 hrL. The egg hatching was significantly prevented under the shorter day length of less than 4 hrL as well as in the darkness.