Expression of multicellular behaviour (rdar morphotype) is a characteristic of wild-type Salmonella typhimurium strains. The key target for the regulation of rdar morphotype expression is the agfD promoter. The regulation of two rdar morphotypes, regulated and semi-constitutive (the latter differs from the former by the insertion of A after position -17), by various environmental conditions was studied using transcriptional fusions to the regulated and semi-constitutive agfD promoters by Western blot analysis and phenotypic analysis of the rdar morphotype. AgfD promoter activities were strongly dependent on oxygen tension. Expression maxima were observed in rich medium under microaerophilic conditions and in minimal medium under aerobic conditions. The regulated rdar morphotype was only expressed under conditions of maximal promoter activity. Glucose did not influence rdar morphotype expression, and the two promoters showed no consistent response to pH. In the stationary phase of growth, nitrogen and phosphate depletion were found to be signals that switch on the agfD promoters. In the logarithmic phase of growth, ethanol was the stress signal that enhanced rdar morphotype expression. The results indicate that, although the regulated and semi-constitutive agfD promoters are key factors in the grade of expression of the multicellular behaviour, common signals such as oxygen tension, depletion of nutrients and ethanol vary their levels of expression significantly.