Host cell barriers to the early phase of immunodeficiency virus replication explain the current distribution of these viruses among human and non-human primate species^,,,. Human immunodeficiency virus type 1 (HIV-1), the cause of acquired immunodeficiency syndrome (AIDS) in humans, efficiently enters the cells of Old World monkeys but encounters a block before reverse transcription^,,. This species-specific restriction acts on the incoming HIV-1 capsid^,, and is mediated by a dominant repressive factor^,,. Here we identify TRIM5α, a component of cytoplasmic bodies, as the blocking factor. HIV-1 infection is restricted more efficiently by rhesus monkey TRIM5α than by human TRIM5α. The simian immunodeficiency virus, which naturally infects Old World monkeys, is less susceptible to the TRIM5α-mediated block than is HIV-1, and this difference in susceptibility is due to the viral capsid. The early block to HIV-1 infection in monkey cells is relieved by interference with TRIM5α expression. Our studies identify TRIM5α as a species-specific mediator of innate cellular resistance to HIV-1 and reveal host cell components that modulate the uncoating of a retroviral capsid.