Theodore bar Koni’s description of the sects of the Kantaeans, the Dostaeans and the Nerigaeans in his Ktābā d-’Eskolyon (ed. Scher 1912 [CSCO/Syr. s2 66 = CSCO 68/Syr. 26] 342:6-347:21) is subjected to a systematic reading. The doctrines described must be characterized as Gnostic. Parallels are adduced from Mandaic literature, tradition, and language. It is argued that the section in which Bar Koni describes these various sects constitutes a unity. In addition, it is maintained that Theodore’s report, which originated in Kashkar, Central Mesopotamia, in the last decade of the eighth century, may be considered the oldest independent outside account of the Mandaeans. An English translation of the Syriac text, together with notes on the Mandaisms it contains, is given at the end of the article and serves as the basis for the discussion.