For the purpose of obtaining knowledge on the seasonal succession of Daphnia pulex, collections were made two or three times a month during 1957〜1958 in the freshwater pond, "Tsujiga-ike", near the Ikeda Campus of the Osaka University of Liberal Arts and Education. The results showed that the population was always monocyclic, while MATSUYAMA noticed that the species was dicyclic in the same pond about 20 years ago. So far as my investigation is concerned, daphnids in this pond began to appear towards the end of November, when the temperature fell to 15℃ ; and gradually increased in number thereafter in spite of further drop of temperature, reaching the maximum population at the end of March. Thence they began to decrease rapidly until they entirely disappeared at the beginning of May, when the water temperature rose to 24℃. Male daphnids were first observed in the latter part of February ; and after gradual increase in number, they were found to make up 14 per cent of the total number of individuals towards the end of March. In the middle of March, ephippial females were likewise found abundantly, and at the beginning of April, nearly all the individuals in the pond seems to be turned out to this state. Laboratory experiments were made on the resting eggs in various conditions of ephippia. There was not a single resting egg collected from the pond in autumn to hatch at temperatures of 30℃ and 2℃ respectively, while some of them were found to hatch at 20℃ or 15℃. Those collected in spring could not hatch at 20℃ and 2℃ respectively, though no fewer than 84 per cent of them could hatch at 12℃ in less than a month. When stored in an ice box for six months, a few of them could hatch at 15℃ or 20℃. However, if they were left in a glass to become dry under natural room conditions, they could not hatch. Accordingly, it may be concluded that the hatching ability of these resting eggs is reduced at temperatures above 30℃ and under drying conditions and that the longer is the period of storage, the weaker becomes the hatchability. Daphnids were bred in 250cc of BANTA'S culture medium at different temperatures as well as under other favourable conditions. The results showed that within six hours all the individuals, both adult and young, died at 30℃ and 25℃. At 20℃, however, they become mature within the shortest period of time, producing thereafter parthenogenetic eggs at the shortest intervals and with the largest in number. At 2℃, though some perished, a considerable number of them still survived, and a part of them continued growth within some 40 days, even producing parthenogenetic eggs a few times. According to TAUSON'S experiments, the range of temperature for the life of Daphnia pulex is 10〜30℃, its optimum range for parthenogenesis being 15〜25℃. My experiments as well as field observations, however, show that the living range in the "Tsujiga-ike" population falls between 2℃ and 23℃, its optimum being between 7℃ and 17℃. Generally speaking, it is believed that, as shown by TAUSON'S experiments, the males and resting eggs occur when the temperature changes from high to low degrees. Contrary to his opinion, however, from my results obtained experimenmlly and from field observations, it was confirmed that the males could appear when the temperature changed from low to high. The adult females were found to produce their ephippia without being fertilized by the males, such a way of ephippium production having repeated twice or more. When they were collected from the "Tsujiga-ike" and bred in the laboratory, the ephippial females mostly perished soon after their ephippia were cast off. However, out of 240 individuals examined, 46 again produced ephippia and three consecutively three times, while eight individuals again laid parthenogenetic eggs after they moulted. Therefore, it may be supposed that the remarkable depression of population at the beginning of April.