Cellulose synthase genes (CesAs) encode a broad range of processive glycosyltransferases that synthesize (1-->4)beta-D-glycosyl units. The proteins predicted to be encoded by these genes contain up to eight membrane-spanning domains and four 'U-motifs' with conserved aspartate residues and a QxxRW motif that are essential for substrate binding and catalysis. In higher plants, the domain structure includes two plant-specific regions, one that is relatively conserved and a second, so-called 'hypervariable region' (HVR). Analysis of the phylogenetic relationships among members of the CesA multi-gene families from two grass species, Oryza sativa and Zea mays, with Arabidopsis thaliana and other dicotyledonous species reveals that the CesA genes cluster into several distinct sub-classes. Whereas some sub-classes are populated by CesAs from all species, two sub-classes are populated solely by CesAs from grass species. The sub-class identity is primarily defined by the HVR, and the sequence in this region does not vary substantially among members of the same sub-class. Hence, we suggest that the region is more aptly termed a 'class-specific region' (CSR). Several motifs containing cysteine, basic, acidic and aromatic residues indicate that the CSR may function in substrate binding specificity and catalysis. Similar motifs are conserved in bacterial cellulose synthases, the Dictyostelium discoideum cellulose synthase, and other processive glycosyltransferases involved in the synthesis of non-cellulosic polymers with (1-->4)beta-linked backbones, including chitin, heparan, and hyaluronan. These analyses re-open the question whether all the CesA genes encode cellulose synthases or whether some of the sub-class members may encode other non-cellulosic (1-->4)beta-glycan synthases in plants. For example, the mixed-linkage (1-->3)(1-->4)beta-D-glucan synthase is found specifically in grasses and possesses many features more similar to those of cellulose synthase than to those of other beta-linked cross-linking glycans. In this respect, the enzymatic properties of the mixed-linkage beta-glucan synthases not only provide special insight into the mechanisms of (1-->4)beta-glycan synthesis but may also uncover the genes that encode the synthases themselves.