An Early Ordovician green alga, Receptaculites camacho n. sp., from the San Juan Formation in Talacasto Gorge, San Juan Province, Argentina is a probable ancestor of Receptaculites oweni of the Galena-Kimmswick (Caradocian?) of North America. Analogy with recent calcareous green algae suggests that R. camacho inhabited warm, shallow, marine water. A paleomagnetic reconstruction of mid-Ordovician continental configurations places the North and South American localities of R. oweni and R. camacho in tropical latitudes. The major global oceanic currents for Middle Ordovician are inferred Includes bibliographical references (p. 107-110) An Early Ordovician green alga, Receptaculites camacho n. sp., from the San Juan Formation in Talacasto Gorge, San Juan Province, Argentina is a probable ancestor of Receptaculites oweni of the Galena-Kimmswick (Caradocian?) of North America. Analogy with recent calcareous green algae suggests that R. camacho inhabited warm, shallow, marine water. A paleomagnetic reconstruction of mid-Ordovician continental configurations places the North and South American localities of R. oweni and R. camacho in tropical latitudes. The major global oceanic currents for Middle Ordovician are inferred Fieldiana series has been published as Geological Series by Field Columbian Museum (1895-1909) and Field Museum of Natural History (1909-1943), and as Fieldiana: Geology by Chicago Natural History Museum (1945-1966) and Field Museum of Natural History (1966-)