A noncytopathic type 1a bovine viral diarrhea virus (BVDV) was isolated from a free-ranging yearling female mule deer (Odocoileus hemionus) from northwestern Wyoming (USA). The mule deer was emaciated, weak, and salivating, and Arcanobacterium pyogenes was cultured from lung abscesses. Bovine viral diarrhea virus was isolated from lung, however, BVDV antigen was not detected by immunohistochemistry. The BVDV genotype was determined by reverse transcriptase polymerase chain reaction and the RNA sequences from the 5′UTR and E2 genes compared with sequences of a type 1a BVDV isolated from cattle from the same area as the deer. The sequences from the deer BVDV were distinct from those of the bovine type 1a BVDV, but similar to other bovine type 1a BVDVs. Seventy-four (60%) of 124 sera collected from mule deer in this area had serum neutralizing antibody titers to type 1a BVDV of ≥1:32. The high prevalence of seropositive mule deer and isolation of BVDV suggests that this virus circulates in the mule deer population. The isolate described in this report is the second reported BVDV isolate from free-ranging deer in North America and the first from a mule deer.