Deer mice are polymorphic for electrophoretic hemoglobin phenotypes showing one, two, or three bands. Within the multibanded phenotypes, there is considerable variation in the hemoglobin partitioning, defined as the fraction of total hemoglobin made up by the secondary and tertiary bands. In subspecies sonoriensis, for example, hemoglobin partitionings range from 0.03 to 0.38. The inheritance of partitioning values is under remarkably strict genetic control. The genetic variation is additive and the narrow heritability is close to 1.0. The inheritance data can be modeled in precise detail by postulating multiple-allele polymorphisms at globin regulatory loci. Comparison of simulated versus actual inheritance data demonstrates that the so-called null structural alleles actually produce functional globins.--The genetic controls in Peromyscus may be analogous to those in primates. Unfortunately, the molecular mechanisms effecting the regulation are unknown. Different subspecies of P. maniculatus show strikingly different arrays of partitioning values, but the role of natural selection in maintaining the quantitative polymorphisms remains obscure.