Although claims about explicit and implicit language knowledge are central to many debates in SLA, little research has been dedicated to measuring the two knowledge types (R. Ellis, 2004, 2005). The purpose of this study was to validate the use of the battery of tests reported in Ellis (2005) to measure implicit and explicit language knowledge. Whereas Ellis (2005) tested only second-language (L2) learners (of English), this study tested both L2 and heritage language (HL) learners (of Spanish). Results showed that test scores loaded on a two-factor model, as in Ellis (2005), thereby providing construct validity for the tests, on a population of HL learners who have little explicit knowledge by virtue of the environment in which they acquired Spanish.