We previously reported that zebrafish alpha 1-3fucosyltrasferase 1 (zFT1) was expressed in embryos at the segmentation period, and was capable of synthesizing the Lewis x epitope [Gal beta 1-4(Fuc alpha 1-3)GlcNAc] [Kageyama et.al, J. Biochem., 125, 838-845 (1999)]. In the current study, we attempted to detect the enzyme products of zFT1 in zebrafish embryos. Oligosaccharides were prepared from the zebrafish embryos at 12, 18 and 48 h after fertilization and labelled with a fluorophore, 2-aminopyridine, for highly sensitive detections. Pyridylamino (PA)-oligosaccharides that were alpha 1-3/4fucosidase sensitive and time-dependently expressed at 18 h after fertilization were identified as candidates for the in vivo products synthesized by zFT1. Structures of these oligosaccharides were determined by a combination of exoglycosidase digestions and two-dimensional HPLC sugar mapping to be the biantennary complex-type structures with two Lewis x epitopes: (Gal beta 1-4)(0,1,2)-{Gal beta 1-4(Fuc alpha 1-3)GlcNAc beta 1-2Man alpha 1-6[Gal beta 1-4(Fuc alpha 1-3)GlcNA beta 1-2Man alpha 1-3]}Man beta 1-4GlcNAc, and (Gal beta 1-4)(0,1)-{Gal beta 1-4(Fuc alpha 1-3)GlcNAc beta 1-2Man alpha 1-6[Gal beta 1-4(Fuc alpha 1-3)GlcNAc beta 1-2Man alpha 1-3]} Man beta 1-4GIcNAc beta 1-4GlcNAc. The presence of Lewis x structure of these oligosaccharides together with their expression time suggests that they are products of zFT1. Remarkably, most of these oligosaccharides were free form. Furthermore, we detected an endo-beta-N-acetylglucosaminidase activity in the 18 h embryo. These results suggest that the oligosaccharides synthesized by zFT1 are present in the embryo at the segmentation period in free form, owing to the liberation from glycoproteins with endo-beta-N-acetylglucosaminidase(s) and/or glycoamidase(s).