Abstract: Under two temperatures(20℃ and 25℃)and five culture volumes(50,100,200,400 and 1000 mL),population dynamic and sexual reproduction of Daphnia magna which came from Huaihe river were studied in this paper.The results showed that:(1) At same culture volume,the mature time and the first brood time of D.magna shortened with the increase of temperature.The mature body length of D.magna increased with the increase of culture volume at same temperature.The mature body length of D.magna in the 1000 mL group was(2.54±0.02)mm at 20℃ and(2.71±0.01)mm at 25℃.(2) At the initial stage,the population size could increase rapidly through parthenogenesis.However,population growth of D.magna was evidently limited at lower culture volumes.The maximal population size of D.magna in the 1000 mL group were 4 times at 20℃(2152 ind./1000 mL) and 13 times at 25℃(3085 ind./1000 mL) higher than that in the 50 mL group.It had significant relationship between the maximal population sizes of D.magna and culture volumes(P0.01,n=15).The first brood size of D.magna were(96.0±36.4) ind./50 mL,(82.3±25.1) ind./100 mL,(328.0±28.8) ind./200 mL,(296.7±36.7) ind./400 mL and(290.3±27.0) ind./1000 mL at 20℃,and was(44.3±3.1) ind./50 mL,(64.0±11.4) ind./100 mL,(283.7±5.5) ind./200 mL,(524.3±18.7) ind./400 mL and(525.7±26.8) ind./1000 mL at 25℃,respectively.(3) During the experiment,the males occurred in all the groups.The number of male of D.magna increased with the increase of population size.The number of male of D.magna in the 1000 mL group was obviously higher than that in other culture volumes at 20℃,and the highest male percent reached 36%,and the males was induced in the first broods.The number of male was positively correlated with the maximal population size of D.magna(20℃: P0.01,n=15;25℃: P0.05,n=15).(4) It was totally produced 814 ephippia at 20℃ and 359 ephippia at 25℃ in the 1000 mL group,which produced by the offspring of the mother,while a few ephippia were produced by the mother or no ephippia appeared in the other groups.The percent of empty ephippia among ephippia was 61.18% at 20℃ and 99.16% at 25℃ in the 1000 mL group.During the experiment,the percent of ephippia containing the resting eggs in the 1000 mL group was up to 20% at 20℃.It was likely that lower temperature and population size might be two important factors that caused sexual reproduction and produced the resting eggs in D.magna.