Abstract:Under isolated and mixed cultures of Daphnia magna and Simocephalus vetulus, the effect of temperature and interspeific competition on populaiton dynamics and sexual reproduction of D. magna was investigated. The mixed cultures included three density combinations: A, 7D+3S; B, 5D+5S; C, 3D+7S. The results showed D. magna had a competitive advantage over S. vetulus under all mixed cultures. At 20℃ and 25℃, maximal population densities of S. vetulus were 2.31 and 1.97 times than one of D. magna under isolated cultures, respectively. But population density of S. vetulus was much lower than one of D. magna under mixed cultures, and S. vetulus died away after twenty-five days. Negative relationships between the population densities of D. magna and S. vetulus were significant (C: r=-0.508, n=30, P<0.01; D: r=-0.483, n=30, P<0.01). Males of D. magna occurred in all density combinations during the experiment. The population density of D. magna was positively correlated with its male density (r=0.678, n=24, P<001) under 20℃ and isolated cultures, and males of D. magna were induced at first reproducion. Maximal male density (106 ind.(200ml)-1) and maximal male percentage (36.8%) appeared under 20℃ and isolated culture. The males of D. magna appeared at first reproduction in B and C combinations at 25℃, and the proportions reached 28.2% in B combinations.No ephippia were produced by D. magna under isolated culutres at 25℃ and mixed cultures 20℃, while there were 66 ephippia under mixed cultures at 25℃, among which 51.5% was empty. The results indicated that the interspecific competition induced the production of males and the formation of ephippia in D. magna, and high density of males might promote to switch from parthenogenic females to sexual females in D. magna.