Salmonellardar morphotype is a distinct, rough and dry colony morphology formed by the extracellular interaction of thin aggregative fimbriae (Tafi or curli), cellulose, and other polysaccharides. Cells in rdar colonies are more resistant to desiccation and exogenous stresses, which is hypothesized to aid in the passage of pathogenic Salmonellaspp. between hosts. Here we analyzed the genetic and phenotypic conservation of the rdar morphotype throughout the entire Salmonellagenus. The rdar morphotype was conserved in 90% of 80 isolates representing all 7 Salmonellagroups; however, the frequency was only 31% in a reference set of 16 strains ( Salmonella reference collection C [SARC]). Comparative gene expression analysis was used to separate cis- and trans-acting effects on promoter activity for the 16 SARC strains, focusing on the 780-bp intergenic region containing divergent promoters for the master regulator of the rdar morphotype ( agfD) and the Tafi structural genes ( agfB). Surprisingly, promoter functionality was conserved in most isolates, and loss of the phenotype was due primarily to defects in trans-acting regulatory factors. We hypothesize that transdifferences have been caused by domestication, whereas cisdifferences, detected for Salmonella entericasubsp. arizonaeisolates, may reflect an evolutionary change in lifestyle. Our results demonstrate that the rdar morphotype is conserved throughout the salmonellae, but they also emphasize that regulation is an important source of variability among isolates.