DNAs complementary to the mRNAs coding for the major cotton 48 kD and 52 kD storage proteins have been cloned and used to characterize the principal cotton storage protein gene family. The principal storage proteins are found to emanate from three subsets of genes that share some homology, as shown by common antigenic determinants shared by the proteins themselves, but that are distinguishable by nucleic acid hybridization. A single sequence subfamily of 2.26 kb mRNAs codes for the 69 kD preproteins (precursors to the mature 48 kD proteins) and two sequence subfamilies of 1.96 kb mRNAs each code for 60 kD preproproteins (precursors to the mature 52 kD proteins). Hybrid arrested translation shows that cloned members of these three subfamilies hybridize only with the mRNAs of a single subfamily at moderate criterion. These three subfamilies comprise all of the principal storage protein mRNAs detectable byin vitro translation. With hybridization at low criterion, some homology has been detected between the two 1.96 kb mRNA families, although no homology has yet been detected between the 2.26 kb mRNA family and either of the two 1.96 kb mRNA families.