The effects of the triazole compound paclobutrazol on the mycorrhizal fungi and production of droppers (tuber stalks) in three species of terrestrial orchids of southwestern Australia were investigated. Seedlings of Diuris laxiflora Lindley (bee orchid), Microtis media R. Br. (common mignonette orchid), and Pterostylis sanguinea D. Jones & M. Clements (dark banded greenhood orchid) were cultured symbiotically with specific mycorrhizal fungi in vitro. The mycorrhizal fungi of the study species were grown on media containing paclobutrazol at 0, 1.7, 3.4, 10.2, or 17.0 muM (corresponding to 0, 0.5, 1.0, 3.0, or 5.0 mg L-1). Paclobutrazol at all concentrations evaluated decreased the growth rates of the mycorrhizal fungi of M. media and P. sanguinea below that of the control. However, paclobutrazol did not adversely affect the growth of the mycorrhizal fungus of D. laxiflora, and low concentrations (1.7 or 3.4 muM) stimulated the growth of this fungus. Symbiotic seedlings of the study species were exposed to paclobutrazol at 0, 1.7, 3.4, 10.2, or 17.0 muM. Paclobutrazol at all concentrations evaluated increased dropper (tuber stalk) production in D. laxiflora, but had no effect on M. media and P. sanguinea.This suggests that, for species in which paclobutrazol has no fungicidal effect on mycorrhizal fungi, it has the potential to stimulate early and efficient tuberization of symbiotic orchid seedlings.