Since the first identification of genes for cellulose biosynthesis in Gluconacetob acter xylinus more than 20 years ago, homologous sequences have been identified in the genomes of a large number of bacterial species. The genes for cellulose biosynthesis are organized in an operon consisting of three to four genes and this organization is conserved in most bacteria. The first gene in the operon encodes for the enzyme cellulose synthase and so far this is the only protein with a known function in cellulose biosynthesis. Although genetic analysis demonstrated a role in cellulose biosynthesis for the other genes in the operon, the exact function of the proteins encoded by these genes remains to be determined. Genetic, biochemical, and structural data suggest that the proteins encoded by genes in the operon are assembled as a multiprotein 2complex and the organization of these complexes in the bacterial envelope determines the nature of the cellulose product. Interestingly, cellulose production has not been demonstrated for a majority of the bacteria where genes for cellulose biosynthesis have been identified, but with better assays it may be possible to determine cellulose production in these bacteria and discuss its evolutionary significance.