Confocal laser scanning microscopy and transmission electron microscopy were used to study the inner membrane complex of tachyzoites of Toxoplasma gondii. DiOC6, a lipophilic cationic fluorescent dye used to visualize the endoplasmic reticulum of eukaryotic cells, labeled cytoplasmic structures in a reticulated pattern and the periphery of the nucleus of the host cell. Intracellular and extracellular tachyzoites were stained. Observation of several focal planes showed labeling of the most peripheral region of the protozoan. Reaction product was observed in the outer nuclear membrane, in profiles of the endoplasmic reticulum, and in the inner membrane complex of tachyzoites subjected to the KI-OsO4 technique. Taken together, these observations suggest that the inner membrane complex may represent a specialized region of the endoplasmic reticulum of tachyzoites of T. gondii.