Fresh and wilted samples of Lolium italicum A. Br., Dactylis glomerata L., Medicago sativa L., have been ensiled in laboratory microsilos for the study of microflora evolution. The principal microbial groups (coliforms, proteolytic bacteria, clostridia, lactic acid bacteria, yeasts) have been detected and enumerated, in anaerobic atmosphere, after 8, 15, 22, 29, 36, 180 days of ensiling. Although the number of lactic acid bacteria resulted to the high in all the samples studied, they failed to perform a quick pH decrease and silage spoiling bacteria were not inhibited. This suggests the possibility of a cohabitation between useful and noxious bacteria, not affected by the chemical composition of ensiled plants.