The osmosensitivity presented by spo0A and degU null mutant strains of Bacillus subtilis pointed to their protein products as essential regulators for the osmotic response. This was further investigated by analyzing their transcription activity. The results showed that both spo0A-lacZ and degSU-lacZ were induced by the hypertonic medium. The actual phosphorylation state of these proteins was also analyzed by the bias of two reporter gene promoters activity (abrB-lacZ and degQ-lacZ). The absence of repression of abrB in hypertonic conditions suggested that Spo0A was not phosphorylated while the derepression of degQ promoter suggested that DegU-P was formed. These results were in accordance with the observed absence of sporulation in hyperosmotic media and semi-constitutive osmotolerance of degUh mutant strains known to retain phosphorylated DegU. The failure to secrete proteases and to sporulate in hypertonic media suggested that Spo0A acts through abrB regulation in the prevention of alternate responses. The role of DegU-P as positive regulator of the osmotic response seems to be settled and is discussed.