Aims: To determine the effect of Propionibacterium acidipropionici, alone or in combination with Lactobacillus plantarum, on the fermentation and aerobic stability of wheat, sorghum and maize silages. Methods and Results: The inoculants were applied at 1.0 x 10(6) CFU g(-1). Silages with no additives served as control. Fresh forages were sampled prior to ensiling. Three jars per treatment were sampled on days 2, 4, 8, 16 and 60 after ensiling, for chemical and microbiological analysis. At the end of the ensiling period, the silages were subjected to an aerobic stability test. The P. acidipropionici-inoculated silages had significantly higher levels of acetic and propionic acid than the L. plantarum or P. acidipropionici + L. plantarum-inoculated silages (P < 0.05). Therefore, yeast activity was impaired in the P. acidipropionici-inoculated silages. As a result, P. acidipropionici decreased CO2 production and improved aerobic stability of wheat, sorghum and maize silages. However, the combination of P. acidipropionici + L. plantarum did not improve aerobic stability of the silages. Conclusions: The P. acidipropionici was very effective in protecting the wheat, sorghum and maize silages exposed to air under laboratory conditions, probably because the acidic environment under ensiling conditions is favourable for this micro-organism. Significance and Impact of the Study: The use of P. acidipropionici, as a silage inoculant can improve the aerobic stability of silages by inhibition of yeast activity.