Aims: The effect of the inoculation of maize and sorghum silages with Lactobacillus plantarum (LP) and Lactobacillus buchneri (LB) on the clostridia spore formation during aerobic deterioration has been studied. Methods and results: The crops were ensiled in 30 l jars, without a lactic acid bacteria inoculant (C), and with an LP or LB inocula (theoretical rate of 1 x 106). After 90 days of conservation, the silages were analysed for the chemical and microbiological characteristics and subjected to an aerobic stability test, during which pH, temperature, nitrate, yeast, mould and clostridia spores were measured. Compared to the C and LP silages, yeasts were reduced in the LB silages, resulting in an increased aerobic stability. Clostridia spores, determined by most probable number (MPN) procedure, increased to 6 log(10) MPN g-1 in the C and LP maize silages, whereas they reached 3 log(10) MPN g-1 in C and LP sorghum silages. Conclusions: Clostridia spore count only slightly increased in the LB maize silages after 342 h (2 center dot 59 log(10) MPN g-1), whereas it did not show any increase in the LB sorghum silages for the whole period of air exposure. Significance and impact of the study: The data indicated that clostridia spore outgrowth can take place during silo feedout in aerobic-deteriorated silages and that LB inoculation reduces the risk of clostridia outgrowth after silage opening by increasing the aerobic stability.