We investigated the effects of colonial Microcystis aeruginosa and interspecific competition on the population dynamics and resting egg formation of two cladocerans. In the single-species culture experiments, high M. aeruginosa concentrations favored the population growth of Moina irrasa but strongly inhibited that of Daphnia carinata. Small-bodied M. irrasa was predominant over large-bodied D. carinata regardless of the presence of the colonial M. aeruginosa in the mixed cultures. The per capita cumulative number of ephippia and the percentage of ephippia containing D. carinata resting eggs both increased from under 0 to 10 mg/L M. aeruginosa in the singlespecies culture but decreased in the mixed culture. However, the per capita cumulative ephippia number of M. irrasa in the mixed culture was higher than those in the singlespecies culture at the high M. aeruginosa levels. The effects of M. aeruginosa, interspecific competition, and their combination on the per capita cumulative ephippia numbers of the two cladocerans were significant. The occurrence of ephippia was related to the higher population density of the two species. Our results suggest that interspecific competition inhibits the ephippium production and resting egg formation of the large-bodied cladocerans (D. carinata) in the presence of colonial M. aeruginosa.