Aureobasidium pullulans strain Ach1-1 was recently isolated for its biocontrol effectiveness against Penicillium expansum, the causal agent of blue mold on harvested apples. In the present study, strain Ach1-1 was found to be very effective in controlling P. expansum on apple wounds. For in vitro tests, strain Ach1-1 and P. expansum were cocultured in the presence of apple juice (0 - 5%) using a system preventing direct contact between both agents. The presence of the antagonist greatly reduced germination of conidia at low (0.1, 0.5 and 1%) but not at high (5%) juice concentrations. Germination of previously inhibited conidia at 0.5% apple juice was partially restored in the presence of the antagonist when fresh juice was added at a final concentration of 5%, and completely recovered at both 0.5 and 5% juice concentrations in the absence of the antagonist. These data show that P. expansum conidia are able to germinate when cocultered with strain Ach1-1 in conditions of sufficient rather than limited nutrient availability and that the antagonist does not affect the viability of these conidia, indicating that the inhibitory effect of strain Ach1-1 on conidia germination may be due to a competition for nutrients. Such observation was confirmed in situ since the application of high amounts of exogenous amino acids, vitamins or sugars on apple wounds significantly reduced the protective level of strain Ach1-1 against P. expansum, the most important effect was obtained with amino acids followed by vitamins and then by sugars. The present work provides both in vitro and in situ evidence that the biocontrol activity of strain Ach1-1 against P. expansum essentially relies on competition for apple fruit nutrients, especially amino acids.