The strains of Theiler's murine encephalomyelitis virus, a picornavirus, are divided into two groups according to their neurovirulence after intracerebral inoculation. The highly virulent GDVII strain causes an acute, fatal encephalomyelitis, whereas the DA strain causes a mild encephalomyelitis followed by a chronic inflammatory demyelinating disease associated with viral persistence. Studies with recombinant viruses showed that the capsid plays the major role in determining these phenotypes. However, the molecular basis for the effect of the capsid on neurovirulence is still unknown. In this paper, we describe a large difference in the patterns of infection of primary neuron cultures by the GDVII and DA strains. Close to 90% of the neurons were infected 12 h after inoculation with the GDVII strain, and the cytopathic effect was complete 24 h postinoculation. In contrast, with the DA strain, viral antigens were not detected in neurons until 24 h postinoculation. Infected neurons accounted for only 2% of the total number of neurons, even 6 days after inoculation. No cytopathic effect was visible, and the cultures could be kept for the same length of time as the noninfected controls. Because the neurovirulence of the GDVII strain has been mapped to the capsid, we examined the role of the capsid in this difference of phenotype. We showed, using recombinant viruses, that the capsid was indeed responsible for the pattern of infection observed in vitro, most likely through its role in viral entry. Thus, the levels of neurovirulence of the GDVII and DA strains correlate with their abilities to infect cultured neurons, and this ability is controlled by the capsid.