|Abstract:||Morphological concatenation often triggers phonological processes. For instance, addition of the plural suffix /-@n/ to Dutch nouns causes vowel lengthening in some nouns ([xat] vs. [xa:.t@n] 'hole') due to the weight-to-stress principle. These kinds of processes often apply only to a subset of words -- not all Dutch nouns undergo this process ([kat] vs. [ka.t@n] 'cat'). Nouns need to be lexically indexed as either undergoing this process or not. I investigate how phonological grammar and lexical indexation are learned when learners are confronted with data like this. Based on learnability considerations, I hypothesize that learners acquire a grammar with default non-alternation, so that novel items are treated as non-alternating. I report the results of artificial language learning experiments compatible with this hypothesis, and model these results in a version the Biased Constraint Demotion Algorithm (Prince and Tesar 2004).