Biocontrol agents may compete with pathogens for nutrients and space to delay or prevent decay of fruits after harvest. These mechanisms of biological control have been difficult to study because no method has been available to determine the significance of each of the components of competition. We developed a nondestructive method using tissue culture plates with cylinder inserts containing defusing membrane at one end to study competition for nutrients without competition for space. Other biocontrol mechanisms in which direct contact between an antagonist and a pathogen is not required also can be studied. The method was used to determine the competition between the yeastlike biocontrol agent, Aureobasidium pullulans, and Penicillium expansum for limited nutrients in apple juice during 24 h incubation, simulating a fruit wound. The antagonist depleted amino acids and inhibited germination of P. expansum conidia. Exposing these conidia to fresh apple juice increased conidial germination to the level comparable to that exhibited by conidia which were not exposed to the antagonist. Because the culture plate method was nondestructive, follow-up experiments in an agar diffusion test were conducted. Juice in which the antagonist grew did not inhibit germination of P. expansum conidia that were seeded on the plates. This corroborates findings from the culture plate method that inhibition of the conidia germination resulted from competition for nutrients. The new method can be coupled with existing techniques to improve understanding of antagonist-pathogen interaction for biological control of postharvest diseases.