The objective of this study was to evaluate the effects of wilting (W) and microbial inoculant (I) on the fermentation characteristics and microbial populations of alfalfa silage. Data were analyzed as a 2 × 2 × 6 factorial arrangement with the effects of wilting (absence or presence), microbial inoculant (with and without) and period of fermentation (P) (1, 3, 7, 14, 28 and 56 d), and their interaction in a completely randomized design with 3 replications. Alfalfa was harvested at 56 d of regrowth and chopped into particles of 1.5 cm. Alfalfa was wilted for 6 h in the field before ensiling, in the material submitted to this treatment. The inoculant used was Sil-All 4x4 W.S. (Alltech, Brazil), applied at the rate of 10⁵ cfu/g of fresh forage. After applying treatments the forage was packed in plastic bags of 25 × 35 cm and sealed by using vacuum sealer. Wilting increased the DM content of the fresh alfalfa from 133.9 g/kg to 233.4 g/kg DM and the population of lactic acid bacteria (LAB) from 5.28 log cfu/g to 6.88 log cfu/g before ensiling. The number of enterobacteria and yeasts were affected (P < 0.05) by P, registering a higher number of these microorganisms on the first day of ensiling. The pH and ammonia nitrogen concentration were affected by the interaction W × P (P < 0.05), recording the lowest values on wilted-alfalfa silages from the 7th day of ensiling. There was effect of the interaction W × P (P < 0.05) on the concentrations of lactic, acetic and propionic acids. Except for the first day of fermentation, we observed higher concentrations of lactic acid in the wilted silages, however in both wilted and non-wilted silages the values decreased from the 7th day of ensiling. The concentration of acetic acid in the wilted alfalfa was lower than non-wilted alfalfa in all period of fermentation. The wilting of alfalfa was not enough to promote adequate silage fermentation in presence or absence of microbial inoculant.