The mechanism of biocontrol of brown rot in stone fruit by Penicillium frequentans Westling (Pf909) was investigated using in vitro and in vivo growth assays and a benomyl-resistant strain of Monilinia fructicola (G Winter) Honey (Mf3C). For the in vitro assays, Pf909 and Mf3C conidia were suspended in Czapek-Dox broth, which was amended or not amended with a skin extract of mature peaches. The growth and germination of Pf909 and Mf3C conidia were determined by counting the number of colony-forming units on potato dextrose agar plates, which were amended or not amended with 0.5 g ml−1 benomyl. In some of the assays, germinated Pf909 conidia were used before their exposure to Mf3C conidia. For the in vivo assays, healthy cherries were inoculated with Mf3C conidia before and after applying Pf909 conidia on the cherry surface and the incidence of brown rot was recorded for seven days. Since we found that Pf909 conidia compete with Mf3C conidia for space and nutrients in the different assays, we concluded that competition is the probable primary mechanism of biocontrol of Pf909. © 2017, International Organization for Biological Control (IOBC).