Gangliosides of the C series such as GT3 are polysialylated glycosphingolipids whose synthesis is developmentally regulated. Here we report the expression cDNA cloning and characterization of GT3 synthase that adds the second alpha-2,8-sialic acid to GD3, NeuNAcalpha2-->8NeuNAcalpha2-->3Galbeta1-->4Glc-->Cer, thus forming GT3, NeuNAcalpha2-->8NeuNAcalpha2-->8NeuNAc alpha2-->3Galbeta1--> 4Glc-->Cer. Unexpectedly, the cloned cDNA was found to be identical to the cDNA that encodes GD3 synthase. The newly identified enzyme was therefore named GD3/GT3 synthase (GD3/GT3ST). GD3/GT3ST synthesized GT3 most efficiently when GM3, NeuNAcalpha2-->3Galbeta1-->4Glc-->Cer, was incubated as an acceptor, indicating that GD3/GT3ST is a polysialyltransferase that can transfer more than one sialic acid residue via alpha-2,8 linkage to gangliosides. Moreover, a longer period of incubation of GD3 with GD3/GT3ST produced a significant amount of GT3 and higher polysialogangliosides. Among various cell lines expressing GD3/GT3ST, higher polysialogangliosides including GT3 were detected only in cell lines where the amount of GD3/GT3 mRNA is sufficiently high. The expression of GD3/GT3ST mRNA among human tissues is highly restricted to fetal and adult brains. The GD3/GT3ST gene was found to be located at chromosome 12, region p12. Taken together, these results indicate that C series polysialogangliosides are synthesized by a ganglioside-specific polysialyltransferase, GD3/GT3ST, that is specifically expressed in neural tissues.