Three-dimensional structures are not available for polysaccharide synthases and only minimal information on the molecular basis for catalysis is known. The Pasteurella multocida hyaluronan (HA) synthase, PmHAS, catalyzes the polymerization of the alternating beta1,3-N-acetylglucosamine-beta1,4-glucuronic acid sugar chain by sequential addition of single monosaccharides to the non-reducing terminus. Therefore, PmHAS possesses both GlcNAc-transferase and GlcA-transferase activities. The recombinant Escherichia coli-derived PmHAS enzyme will elongate exogenously supplied HA chains in vitro with either a single monosaccharide or a long chain depending on the UDP-sugar availability. Competition studies using pairs of acceptors with distinct termini (where one oligosaccharide is a substrate that may be elongated while the other cannot) were performed here; the lack of competition suggests that PmHAS contains at least two distinct acceptor sites. We hypothesize that the size of the enzyme’s acceptor binding pockets corresponds to the size of the smallest high efficiency substrates thus we tested the relative activity of a series of authentic hyaluronan oligosaccharides and related structural analogs. The GlcA-transferase site readily elongates (GlcNAc-GlcA)₂ while the GlcNAc-transferase elongates GlcA-GlcNAc-GlcA. The minimal size oligosaccharides elongated with high efficiency both contain a trisaccharide with two glucuronic acid residues which enabled the identification of a synthetic, artificial acceptor for the synthase. PmHAS behaves as a fusion of two complete glycosyltransferases, each containing a donor site and an acceptor site, in one polypeptide. Overall, this information advances the knowledge of glycosaminoglycan biosynthesis as well as assists the creation of various therapeutic sugars for medical applications in the future.