Tachyzoites of Toxoplasma gondii were located inside the nucleus of both skeletal muscle cells infected in vitro and peritoneal exudate cells collected from infected mouse in vivo. Ultrastructural analysis demonstrated that T. gondii invades the nucleus of host cells by the parasite apical region and with constriction of its body. We noted that the rhoptry, a secretory organelle of the parasite that is involved in the host cell invasion mechanism, was empty in the intranuclear T. gondii. The parasites were found in the nuclear matrix without evidence of the vacuolar membrane. Frequently, new parasites invaded host cell nucleus, which was already infected. The significance of this nuclear invasion could reflect an alternative route of T. gondii for its transitory survival or an escape mechanism from the host immune response during the in vivo infection (or both).