We report on the morphology and occurrence of the “mystax,” an overlooked cephalic structure found in males of certain Notostraca. The mystax is a conspicuous, protuberant, dark-brownish sclerodermal stripe located on the anteroventral flange of the carapace. Histological sections show the mystax as an expansion of the exoskeleton formed by the exocuticle. Examination of exuviae revealed that the old mystax remains intact in the shed cuticle. In some Triops populations of México and Japan, large individuals bearing ovisacs exhibit a “pseudomystax,” which consists of a dark-brownish band in nearly the same position as the mystax, but it does not form a protuberant stripe. A total of 661 males and 6402 ovisac-bearing individuals from four continents were examined. The mystax was found in males of Triops australiensis from Australia, T. granarius from Japan, Triops sp. from North America, Triops sp. from Africa, and Triops sp. from Asia. The mystax is absent in all ovisac-bearing individuals of all species analyzed and in all males of Triops cancriformis (Africa, Europe, and Japan), and in all males of the genus Lepidurus. Practical and systematic implications of the mystax are discussed.