An electrophoretic polymorphism for beta-galactosidase has been identified among common inbred strains of mice. It is inherited as a single Mendelian factor with two alleles showing codominant expression. This structural gene, Bge, is closely linked (0/163 recombinants) with the Bgs site on chromosome 9 which regulates systemic levels of beta-galactosidase. The distribution of electrophoretic and activity phenotypes among inbred strains is not concordant, indicating that they result from separate mutations. Three aspects of beta-galactosidase realization, its structure, systemic regulation and developmental program, are now known to be organized in close proximity on chromosome 9. Considered in conjuction with evidence from other mammalian systems, this suggests that the mammalian genome, like Drosophila, is organized into large functional units in which relevant regulatory and developmental information is closely associated with individual structural genes.