A number of Mayan languages display optional verbal agreement (Henderson 2008, England 2011). Focusing on novel data from Santiago Tz’utujiil (ST), we demonstrate that this optionality is not reducible to phonological or morphological factors. Rather, the source of optionality is in syntax. Regarding optional agreement, there is a syntactic distinction between external arguments (EA) and internal arguments (IA). Only IAs show optional agreement; EAs in contrast must be indexed for person/number on the verb. We provide an analysis in which surface optionality is a result of two possible syntactic derivations.