Vitellin (Vn) and vitellogenin (Vg) of the beet armyworm, Spodoptera exigua, were identified and characterized. Both were similar in molecular size (a 180 kDa) and showed cross-immune reactivity. Vn was purified by ammonium sulfate (25~50%), size-exclusion chromatography (G-100) , ion-exchange chromatography (DEAE), and affinity chromatography(concanavalin-A). The purified Vn was used for raising its antibody. The raised Vn-antiserum reacted specifically with Vn and Vg proteins in S. exigua, but had a slight cross-reactivity with P22O, probably lipophorin large subunit. The Vn-antiserum did not react to Vns of Bombyx mori, Plutella xylostella, and Drosophila melanogaster. Using this antiserum, Vg biosynthesis could be assessed with adult development of S. exiua. Vg could be detected in the hemolymph and the fat body as early as 5h before adult emergence. Fenoxycarb, a juvenile hormone analog, could induce Vg biosynthesis.