The effect of competition on species that coexist with similar ecological niches is an important theme in ecology. Furthermore, species displacement by introduced or invaded species is also an important environmental problem for biological control and conservation ecology. We tested whether two species of phytoseiids could coexist in closed cages with ample quantities of the extraguild prey species Carpoglyphus lactis (L.). Three species of phytoseiid mites—Amblyseius eharai Amitai & Swirski (a species native to China), Amblyseius swirskii (Athias-Henriot) and Neoseiulus cucumeris (Oudemans) (both species were introduced from outside of China)—were tested under experimental conditions (25 ± 1° C, 90 ± 5% relative humidity, and a photoperiod of 14: 10 [L: D] h). With extraguild prey, we found that the numbers of a single population of each phytoseiid species (initial density of 10 females per cage) reached a plateau between 18 and 25 d after introduction into the experimental cages, suggesting that density-dependent factors were operating. In closed environments, one of these density-dependent factors might be cannibalism by these species. With regression analyses, Lotka–Volterra equations estimated the rate of population increase (r) and the carrying capacity (K) of each species with the data from observations on population dynamics. We next observed the interactions of two phytoseiid species with abundant extraguild prey. In all species combinations, one species went extinct and the other increased in population size, despite the availability of sufficient extraguild prey, suggesting some type of competition must have caused the extinctions. We suggested that intraguild predation is the most plausible hypothesis to explain the results.