The acquisition of Spanish null pronouns is an optimal domain for comparing the predictions of generativist vs. probabilistic approaches to language acquisition. This paper presents two studies on the acquisition of null subjects by English adult learners of Spanish as a second language. The first investigates a low frequency construction in which the antecedent of the pronoun is a quantifier, and the distribution is regulated by a principle of UG. The second looks at a high frequency context,where the distribution of the null pronoun depends on whether it is interpreted as focus or as discourse topic. The data indicate early mastery, and no development in the case of the low frequency quantifier construction, and gradual acquisition for the distribution of pronouns in discourse. These findings lend support to grammatical as opposed to probabilistic approaches to language learning.