Acetic acid bacteria were isolated from maize silages and from samples of maize silage exposed to air. The isolates apparently belonged to the genus Acetobacter. By inoculating maize silage with strains of acetic acid bacteria isolated from silage and by monitoring the development of the microbiota of samples exposed to air it was demonstrated that acetic acid bacteria can be responsible for the onset of aerobic deterioration of maize silage. However, acetic acid bacteria and yeasts often developed simultaneously in uninoculated silage samples exposed to air. In all experiments ethanol was oxidized to acetic acid followed by a rapid oxidation of lactic and acetic acids when ethanol was depleted.