Toxoplasma gondii (T. gondii) is one of the most common opportunistic infections affecting the central nervous system (CNS) in AIDS patients. Disease results from a reactivation of a latent infection in the brain resulting in a severe and necrotizing encephalitis. In this study we infected a primary culture from human fetal brain with T. gondii and studied the behavior of both the active and latent stages in this culture system. We found that the active (tachyzoite) stage of T. gondii can infect both astrocytes and neurons. However, a higher percentage of astrocytes were infected than neurons. Additionally, astrocytes were found to support more replication of T. gondii than did neurons. Both astrocytes and neurons also supported the cyst stage, found in the latent infections. These data indicate that astrocytes are the host cells supporting most of the replication of T. gondii in the brain in reactivated infections, but both host cell types may be able to support the cyst stage in latent infections. However, evidence indicates that cysts formed in astrocytes may be distinct from neuronal cysts. These findings may have relevance to reactivation of latent T. gondii infections in AIDS patients.