This paper offers an in-depth look at roots and verb stem morphology in Chuj (Mayan) in order to address a larger question: when it comes to the formation of verb stems, what information is contributed by the root, and what is contributed by the functional heads? I show first that roots in Chuj are not acategorical in the strict sense (cf. Borer 2005), but must be grouped into classes based on their stem-forming possibilities. Root class does not map directly to surface lexical category, but does determine which functional heads (i.e. valence morphology) may merge with the root. Second, I show that while the introduction of the external argument, along with clausal licensing and agreement generally, are all governed by higher functional heads, the presence or absence of an internal argument is dictated by the root. Specifically, I show that transitive roots in Chuj always combine with an internal argument, whether it be (i) a full DP, (ii) a bare pseudo-incorporated NP, or (iii) an implicit object in an antipassive. In the spirit of work such as Levinson (2007, 2014), I connect this to the semantic type of the root; root class reflects semantic type, and semantic type affects the root’s combinatorial properties. This work also contributes to the discussion of how valence morphology operates. In line with works such as Alexiadou, Anagnostopoulou & Schäfer (2006), I argue that valence morphology applies directly to roots, rather than to some ‘inherent valence’ of a verb.