A significant body of theoretically motivated research has addressed the role of Universal Grammar (UG) in the nonnative acquisition of morphosyntax and properties of the syntax-semantics interface, but very little research has addressed the role of phonological principles of UG in nonnative language acquisition. Turkish has a regular and pervasive system of vowel harmony for which classroom second language (L2) learners receive explicit instruction and abundant input; however, there are also cases of non-canonical vowel harmony in Turkish, for which classroom learners receive no instruction and rather little input. In this study, we show that English-Turkish L2ers come to exhibit sensitivity to the "No Crossing Constraint" of UG (Goldsmith, 1976; Hammond, 1988) when calculating non-canonical vowel harmony in the context of underlyingly pre-specified non-velarized laterals (i.e. "light" [l]), despite the poverty of the stimulus and potentially misleading effects of classroom instruction and standard Turkish orthography. We argue that this supports the view that nonnative phonological development is guided by (at least one principle of) UG.