A prerelease assessment of impact by a potential biological control agent, Floracarus perrepae Knihinicki and Boczek, on the invasive weed, Lygodium microphyllum (Cav.) R. Br., was conducted in a 2-yr field study in their native range—Australia. Thirty-two pairs of test plants were planted in a field plot with two levels of shade, with one plant in each pair treated biweekly with the miticide abamectin. The mite caused a significant reduction in biomass of above ground stems and leaves and below ground roots and rhizomes. The mean leaf longevity was significantly longer for the treated versus the mite infested untreated plants. Populations of native predator mites were low throughout the study; however, the mite pathogen Hirsutella thompsonii Fisher was common in the second year of the study, but neither reduced the impact of F. perrepae. Based on its potential to cause significant damage to L. microphyllum under field conditions in the native range and extremely narrow field host range, F. perrepae is an excellent candidate for biological control of this invasive fern in Florida.