Female fruit flies, Drosophila melanogaster, lay their eggs on decaying plant material. Foraging fly larvae strongly depend on the availability of dietary microbes, such as yeasts, to reach the adult stage. In contrast, strong interference competition with filamentous fungi can cause high mortality among Drosophila larvae. Given that many insects are known for employing beneficial microbes to combat antagonistic ones, we hypothesized that fly larvae engaged in competition with the noxious mould Aspergillus nidulans benefit from the presence of dietary yeast species, especially when they are associated with increasingly species rich yeast communities (ranging from one to six yeast species per community). On a nutrient‐limited fruit substrate infested with A. nidulans, both larval survival and development time were positively affected by more diverse yeast communities. On a mould‐free fruit substrate, merely larval development but not survival was found to be affected by increasing species richness of dietary yeasts. Not only yeast diversity had an effect on D. melanogaster life‐history traits, but also the identity of the yeast combinations. These findings demonstrate the importance of the structure and diversity of microbial communities in mutualistic animal–microbe interactions.