C. decurvata sp. n. (holotype Q ; Fla. St. Coll. Arthropods, Gainesville, Florida, USA) is described from 15 males collected at Alta Gracia, Cordoba Province, Argentina, in January 1979. The new sp. is similar to C. januaria (Hag.) but differs in having downcurved nearly hairless male cerci, wider green stripes on the thorax, and the radial sector vein (Rs) forked asymmetrically. A key to adults of the 7 described spp. of Castoraeschna is presented.