Abstract
A new whiptailed stingray, Himantura chaophraya, is described from three specimens caught in the Mae Nam Chao Phraya of Thailand. The new species, reportedly attaining 500kg or more, belongs to a species group found mainly in fresh water and characterized by large body size, strongly projecting snout, broad but thin oval disc, small eyes, black marginal coloration on ventral surface of disc, very narrow tail base, 158-164 pectoral radials, and 21 spiral valve turns. The species group is known from India, Borneo and New Guinea and probably also occurs in the Mekong River, but the only specimens available in museum collections are from the Chao Phraya and the Mahakam River basin in eastern Borneo (Kalimantan, Indonesia).
