T antigen (Galbeta1-3GalNAcalpha1-Ser/Thr), the well-known tumor-associated antigen, is a core 1 mucin-type O-glycan structure that is synthesized by core 1 beta1,3-galactosyltransferase (C1beta3GalT), which transfers Gal from UDP-Gal to Tn antigen (GalNAcalpha1-Ser/Thr). Three putative C1beta3GalTs have been identified in Drosophila. However, although all three are expressed in embryos, their roles during embryogenesis have not yet been clarified. In this study, we used P-element inserted mutants to show that CG9520, one of the three putative C1beta3GalTs, synthesizes T antigen expressed on the central nervous system (CNS) during embryogenesis. We also found that T antigen was expressed on a subset of the embryonic hemocytes. CG9520 mutant embryos showed the loss of T antigens on the CNS and on a subset of hemocytes. Then, the loss of T antigens was rescued by precise excision of the P-element inserted into the CG9520 gene. Our data demonstrate that T antigens expressed on the CNS and on a subset of hemocytes are synthesized by CG9520 in the Drosophila embryo. In addition, we found that the number of circulating hemocytes was reduced in third instar larvae of CG9520 mutant. We, therefore, named the CG9520 gene Drosophila core 1 beta1,3-galactosyltransferase 1 because it is responsible for the synthesis and function of T antigen in vivo.