An oocyte-yolk protein was purified by double-step chromatography from amphioxus ovaries. The purified protein appeared to exist as a homodimer of approximately 320kDa in native polyacrylamide gel electrophoresis(PAGE), and was reduced to a single monomer of approximately 160 kDa in sodium dodecyl sulfate-PAGE(SDS-PAGE). The protein was characterized as a phospholipoglycoprotein by native PAGE and staining of gels for phosphorus with methyl green, for lipids with oil red O and Sudan black B, and for carbohydrates using periodic acid Schiff reagent. In addition, the amino acid composition of the oocyte-yolk protein was generally similar to that of vitellogenins(Vgs) isolated from different phyla of animals including both vertebrates and invertebrates. The purified phospholipoglycoprotein is thus considered as putative amphioxus Vg.