The ba construction is central to the study of Mandarin grammar. It has received many attempts at analysis and comes up frequently as a syntactic test in discussions of other phenomena. Yet, not even its part of speech has ever been convincingly established. This paper presents the case for treating ba as a verb, considering both language-internal arguments and arguments from universal properties of parts of speech. These arguments are intended to have cross-theoretic validity. On the basis of the conclusion that ba is a verb, an analysis is developed within the framework of Lexical Functional Grammar. On this analysis, ba selects for a subject, an object, and a complement clause, and further stipulates that its object controls the TOPIC function of its complement clause. This analysis is shown to account for both the core data and the data which is problematic for other analyses. Finally, the analysis is confirmed by evidence from telecity effects in the bas construction, universal properties of verbs and prepositions, and its compatibility with the known historical development of the construction.