Salmonella typhimurium strains MAE52 and MAE32 display a constitutive multicellular morphotype mediated by the expression of the agfD operon. In those strains, the role of flagella in the formation of various modes of multicellular behavior was investigated. Flagella were not required for the formation of the multicellular morphotype (rdar) on plates. However, visual examination showed that the global behavior of the bacterial community on air-liquid, surface-liquid or cell-cell-liquid interfaces changed in the absence of flagella. No differences in the local cell-cell interactions were observed at the microscopic level. Using Western blot analysis, no co-regulation of flagella and thin aggregative fimbriae, an extracellular component of the multicellular morphotype, was observed either on plates or in standing culture. In a mutant lacking flagella and thin aggregative fimbriae, the contribution of the latter to the multicellular morphotype was dominant. We concluded that independently regulated genes can act in an additive fashion to confer a pronounced multicellular behavior.