The human chemokine receptor hCXCR-4 serves as a coreceptor for T-cell-tropic (T-tropic) and dual-tropic strains of human immunodeficiency virus type 1 (HIV-1). We have isolated a homolog of hCXCR-4 from a murine T-cell cDNA library and have examined its ability to function as an HIV-1 coreceptor. mCXCR-4 was found to be 91% identical to the human receptor at the amino acid level, with sequence differences concentrated in extracellular domains. Surprisingly, coexpression of both hCD4 and mCXCR-4 on either simian or murine cell lines rendered them permissive for HIV-1-induced cell fusion, indicating that mCXCR-4 is a functional HIV-1 coreceptor. As with hCXCR-4, coreceptor function was restricted to T-tropic and dual-tropic HIV-1 strains. Ribonuclease protection analysis indicated that mCXCR-4 mRNA was expressed in only two of six murine cell lines tested. In contrast, Northern blot analysis of human and mouse tissues revealed that CXCR-4 is widely expressed in both species in vivo. Overall, these data suggest that the reported lack of susceptibility of hCD4+ murine cells to HIV-1 infection in vitro is, at least in part, due to a lack of mCXCR-4 expression rather than a lack of coreceptor function.