Although chemical glycosylation has certain advantages compared with enzymatic glycosylation in respect of its high flexibility and wide applicability, the reaction processes are complicated in many cases because the chemical reactions require multiple protection/de-protection steps. On the other hand, enzymatic glycosylation using glycosyltransferases is a single step process with high position- and anomer-selectivity and reaction yield. However, glycosyltransferases available for this purpose had been very limited and costly. Since then, progress has been made in the search of prokaryotic glycosyltransferases, increasing the types of enzymes that can be employed in synthesis and modification of sugar chains. Moreover, various studies have been making sugar nucleotides, which are donor substrate for the glycosyltransferases, less and less expensive. Sialic acids are present in a variety of glycoproteins and glycolipids, often at the non-reducing termini of carbohydrate chains. It has been demonstrated that sialic acids play very important roles in various biological and physiological events. Ample supply of sialosides and sialyl-glycoconjugates is indispensable to the study of their biological functions in detail. Transfer of sialic acids by sialyltransferases to appropriate substrates in the final step under a mild reaction condition can prepare these materials in quantity. Therefore, one of the most important tasks in the study of glycobiology is to provide a large amount of bacterial sialyltransferases with diverse characteristics at low prices.