Using a modified rotating-disk reactor to sparge oxygen to Staphylococcus epidermidis cultures, we found that oxygen negatively regulates biofilm development by influencing the activity of sigma(B). Under anaerobic conditions, increased sigma(B) activity activates icaADBC, which encodes enzymes responsible for polysaccharide intercellular adhesin synthesis, by repressing transcription of the negative regulator icaR.