This paper addresses the issue of the application of universal grammatical principles in adult grammars. Specifically, I show that the application of vowel harmony in Modern Hebrew, which is not part of the native phonological system, is universally motivated in certain areas of the lexicon. The goals of this paper are twofold. First, I demonstrate that vowel harmony, a non-native process in Modern Hebrew nevertheless applies to certain areas of the lexicon, supporting an approach that vowel harmony is universally motivated (UG). Second, I demonstrate that UG may apply in a (seemingly) sporadic manner even in what appear to be stable grammatical systems.