The mouse Igk-VSer gene encodes an immunoglobulin kappa light chain variable region which gives rise to two phenotypic polymorphisms of mouse kappa chains. The nucleotide sequences of coding and flanking regions of the Igk-VSerc and Igk-VSerd alleles found in recently inbred strains of wild mice are compared with those of the Igk-VSera and Igk-VSerb alleles described previously. Results suggest that the gene is evolving randomly and that framework 2 and complementarity determining region 2 are preserved, presumably for overall light chain structure. Results indicate that all four alleles have an octamer motif upstream of the gene which should be functional and allow prediction of whether or not the product of the germ line gene will be detectable as either the IB-peptide or Ef1a phenotypic polymorphism. Southern hybridization of genomic DNA using as probe a 1-kb Xba I-Xba I fragment located approximately 4 kb upstream of the BALB/c Igk-VSerb coding region demonstrated the presence of homologous DNA in mice bearing the Igk-VSera allele and absence from mice bearing the Igk-VSerc and Igk-VSerd alleles. Nucleotide sequence comparison of BALB/c and SK/CamRk (Igk-VSerd) DNA in this region demonstrated that BALB/c contained an insertion 2.4 kb in length which was absent from SK/CamRk. Both strains contain DNA homologous to the reverse complement of the mouse Bam5 repetitive element at the point of the insertion, with BALB/c containing approximately 70 nucleotides more of the element than SK/CamRk. Surprisingly, the strains containing DNA related to the Xba I-Xba I probe are not those determined to be the most similar by nucleotide sequence comparisons and by the Phylogenetic Analysis Using Parsimony program. The evolutionary relationship of the alleles and a possible basis for the inconsistency presented by the Xba I-Xba I fragment-related DNA are discussed.