Competition was investigated between two pest earth mite species, Penthaleus major Dugés and Halotydeus destructor Tucker, which occur sympatrically and are major pests of southern Australian pastures and crops. Three geographically separate pasture sites within Victoria were sampled monthly. Abundance patterns were similar across sites in that there was a marked increase in numbers of H. destructor in spring. Field competition plots were set up on pasture. In the first season, H. destructor had a negative effect on P. major but not vice versa. During the summer, diapause and ensuing generation, density-dependent (intraspecific) competition was evident in P. major but not in H. destructor. The interspecific asymmetry was switched in the following season, with P. major dominating most plots and having a negative effect on the reproductive output of H. destructor. These findings indicate that suppression or eradication of one species of earth mite from pastures may result in another species increasing in relative abundance.