After biological control of Tetranychus urticae using Phytoseiulus persimilis, a latent mite pest, Eotetranychus asiaticus, was found on strawberries growing in a plastic greenhouse in western Japan. To determine whether the release of P. persimilis, an exotic natural enemy of T. urticae, enhanced the occurrence of the latent pest, the efficiency of P. persimilis in controlling E. asiaticus was compared with the efficiency of two indigenous phytoseiid mites, Amblyseius californicus and A. womersleyi, under laboratory conditions. Phytoseiulus persimilis and A. californicus attacked not only T. urticae but also E. asiaticus. However, the predators produced very few eggs and their offspring rarely reached adulthood when fed on E. asiaticius, so their potential as control agents for the latent mite appears to be low. In contrast, A. womersleyi feeding on E. asiaticus reproduced as well as when fed on T. urticae, and exterminated both species of spider mite. This suggests that A. womersleyi has greater potential as a biological control agent in crops where both species may occur.