fi-Conglycinin, an abundant storage protein in soybean (Glycine max (L.) Merr.) seeds, is a trimeric protein consisting of various isomers containing the three subunits, c(, e and ft. Accumulation of the fl-subunit is unique because it appears to be regulated by a variety of developmental and environmental signals. In this paper we describe the isolation and characterization of a genomic clone encoding the fl-subunit of fl-conglycinin. The genomic clone was characterized by restriction-enzyme mapping and partial DNA sequence analysis, by immunoprecipitation of a hybrid-selected invitro translation product, and by RNA blot hybridization reactions. An mRNA of approx. 1700 nucleotides hybridized to an internal 2-kilobase (kb) region of this 4.4-kb cloned DNA restriction fragment and was translated to yield a polypeptide with an approximate molecular weight of 48 kilodalton. This polypeptide is immunoprecipitable by antibody against fl-conglycinin and is of appropriate size to represent the precursor polypeptide of the fl-subunit. When this sequence was used as a probe in RNA blot hybridization experiments, the fl-gene transcript was first detected by stage K and accumulated through stage O during soybean seed development, coincident with appearance of the fl-subunit. Partial DNA sequence analysis of the 5 / end of the gene confirmed that the isolated gene encoded a fl-subunit, based upon the