ABSTRACT. Toxoplasma gondii, the agent causing toxoplasmosis, is an obligate intracellular protozoan parasite.A calcium signal appears to be essential for intracellular transduction during the active process of host cell invasion. We have looked for a Ca 2+ -transport ATPase in tachyzoites and found Ca 2+ -ATPase activity (11-22 nmol Pi liberated/mg protein/min) in the tachyzoite membrane fraction. This ATP-dependent activity was stimulated by Ca 2+ and Mg 2+ ions and by calmodulin, and was inhibited by pump inhibitors (sodium orthovanadate or thapsigargin). We used cytochemistry and X-ray microanalysis of cerium phosphate precipitates and immunolabelling to find the Ca 2+ , Mg 2+ -ATPase. It was located mainly in the membrane complex, the conoid, nucleus, secretory organelles (rhoptries, dense granules) and in vesicles with a high calcium concentration. Thus, Toxoplasma gondii possesses Ca 2+ -pump ATPase (Ca 2+ , Mg 2+ -ATPase) as do eukaryotic cells.