Imam al-Mansur al-Qasim b. Muhammad (d. 1029/1620) was the founder of the Qasimi dynasty as well as the ancestor of the Hamid al-Din imams. He was a towering intellectual and political figure, who held distinctive theological and legal opinions, and can be considered the last great scholar among the later Zaydi imams. This presentation is based on a manuscript treatise I have co-edited and will consist of detailing and analyzing a polemical exchange over religious doctrine and Sufism between Imam al-Qasim and a pro-Ottoman opponent. Titled "Kitab Hatf anf al-afik fi jawabihi `ala al-Kamil al-mutadarik," the treatise is a commentary on a long poem. It sheds light on later Zaydi theology, the sect’s attitude toward Sufism and the Ottomans, who were then seeking to control Yemen. As such, it represents an important source for the political and intellectual history of 11th/17th-century Yemen.