Most second language (L2) researchers and teachers would agree that input, often defined as the language a learner hears or reads, plays an important role in L2 learning. There is a great deal of research investigating which types of input are most beneficial for learning, how learners process and internalize input (e.g., Schmidt, 2001), and how input relates to language learning outcomes. However, research investigating the role of input in learning L2 phonology, particularly in instructed settings, is conspicuous by its absence (although research in this vein with native speakers has proved fruitful, as shown by Bybee, 2002). In this study, the authors extend their research to the domain of phonology by investigating the relationship between teacher input and student output with respect to the voiced interdental fricative, a difficult target for French learners of English. Their analyses of the input-output relationship were informed by frequency-based approaches to language learning and use. (Contains 3 figures and 3 footnotes.)