Bacillus anthracis appears to be the most molecularly homogeneous bacterial species known. Extensive surveys of worldwide isolates have revealed vanishingly small amounts of genomic variation. The biological importance of the resting-stage spore may lead to very low evolutionary rates and, perhaps, to the lack of potentially adaptive genetic variation. In contrast to the overall homogeneity, some gene coding regions contain hypervariability that is translated into protein variation. During marker analysis of diverse strains, we have discovered a novel ca. 750-nucleotide open reading frame (ORF) that contains in-frame, variable-number tandem-repeat sequences. Four distinct variable regions exist within vrrB, giving rise to 11 distinct alleles in eight different length categories among B. anthracis strains. This ORF putatively codes for a 241- to 265-amino-acid protein, rich in glutamine (13.2%), glycine (23.4%), and histidine (23.0%). The variable-region amino acids of the vrrB ORF are strongly hydrophilic. Coupled with putative transmembrane domains flanking the variable regions, this suggests a membrane-anchored cytosolic or extracellular location for the putative protein. Sequence analysis of the complete ORFs from three Bacillus cereus strains shows maintenance of the ORF across species boundaries, including strong conservation of the amino acid sequence and the capacity to vary among strains. The presence of 11 different alleles of the vrrB locus is in stark contrast to the near homogeneity of B. anthracis. Evolution of hypervariable genes can negate the lack of genetic variability in species such as B. anthracis and provide select rapid evolution in other more variable species.