Haloarchaeal flagella are composed of a number of distinct flagellin proteins, specified by genes in two separate operons (A and B). The roles of these flagellins were assessed by studying mutants of H. salinarum with insertions in either the A or the B operon. Cells of the flgA− mutant produced abnormally short, curved flagella that were distributed all over the cell surface. The flgA2− strain produced straight flagella, mainly found at the poles. The flgB− mutant had flagella of the same size and spiral shape as wild-type cells, but these cells also showed unusual outgrowths, which appeared to be sacs filled with basal body-like structures. In broth cultures of this mutant, the medium accumulated flagella with basal body-like structures at their ends.