The article focuses on the question: Was Julian of Eclanum (c. 380–454) right in accusing Augustine (354–430) of still being a Manichaean, based on his view of sexual concupiscence and the transmission of (original) sin? In order to find an answer to this (still hotly debated) question, a sketch of Augustine’s acquaintance with Manichaeism is first provided. Thereafter follows the (first ever) overview of the Manichaean doctrines of the origin of sexual concupiscence, its distinctive features, and its role in the transmission of sin. The third part of the article focuses on the essentials of Augustine’s views of sexual concupiscence and the transmission of original sin, in particular as they were expounded (and further developed) in his controversy with the ‘Pelagian’ bishop, Julian of Eclanum. It is concluded that, in particular, Augustine’s stress on the ‘random motion’ (motus inordinatus) as typical of the sinfulness of sexual concupiscence, is strikingly similar to Manichaean views on the subject. In this respect, then, Julian seems to be right. Finally, some preliminary remarks are made on early Jewish and Jewish-Christian views of sexual concupiscence and (original) sin which may have influenced not only Mani and his followers, but also Augustine and his precursors in the tradition of Roman North Africa. The current article is the first in a series of two essays on the topic.