A phosphate-solubilising isolate of Penicillium radicum (sp. nov.) was used to inoculate wheat (Triticum aestivum cv. Dollarbird) in a glasshouse experiment and a field trial, using low pH soils at 5 levels of phosphate application. When data for all phosphate application levels were combined, inoculation by P. radicum resulted in 14% increases in wheat yield in the field trial and increases in both phosphate uptake (10%) and yield (9%) in the glasshouse. In the glasshouse, the response to inoculation was higher at the nil phosphate application level (17%) than for the combined phosphate application levels (9%). This negative interaction indicates that growth promotion could be partially due to soil phosphate solubilisation by the fungus, with a greater response for soils with lower available P. However, other plant-growth promotion mechanisms may also be involved.