The polymorphism of two serologically detected loci in the I region was assessed in wild mice trapped in six separate locations in Texas. About 100 mice were typed by the two-stage microcytotoxicity test with a battery of alloantisera detecting 10 private, semi-private, or public specificities in either the I-A or I-EC loci. This survey has revealed that three different private I-region specificities are present among wild mice in Texas at frequencies characteristic of stable polymorphisms. Two of these private specificities, Ia-1.2 and Ia-1.4, were also present in the descendents of mice trapped in Michigan, indicating that these alleles are widely distributed among wild mice in the United States. A comparison of the distribution of I region specificities with K or D region specificities indicated that linkage disequilibrium among these H-2 loci was a rare, but detectable event. Finally, breeding studies with wild, male mice indicate that about 90% of the wild males are heterozygous for H-2 antigens.