Soybean [Glycine max (L.) Merr.] seed protein is deficient in the essential sulfur-containing amino acids, methionine, and cysteine. Transgenic approaches to increase coding for these amino acids have not been particularly fruitful. Little is known about the relative importance of S stored in vegetative tissues for seed protein synthesis. This study was conducted to secure data on sulfur (S) uptake, distribution, and mobilization patterns in the reproductive phase of soybean grown under S sufficient conditions. Soybean plants were grown in hydroponic culture and pulsed with 35S labeled SO4 at several discrete stages during the reproductive phase of development. Sequential harvest of plants indicated where and how S is stored and mobilized during reproduction in soybean. Leaves supplied developing soybean seed with approximately 20% of its S requirement. Distribution of newly acquired S within plants changed through reproductive development; however, leaves mobilized S with a similar efficiency through R5.5. Expanding leaves take up disproportionately large quantities of S; however, leaf tissue seems dependent on newly acquired S, and is unable to utilize stored SO4. Pods play an important role in S storage and mobilization to seed. In contrast to the leaf fraction, expanding pods were able to utilize stored S. Pods and seeds seem dependent on mobilized S. This study demonstrates that developing soybean seeds are dependent on S that has been mobilized from other plant tissues.