The obligatory use of the preposition a with animate, specific direct objects in Spanish ("Juan conoce a Maria" "Juan knows Maria") is a well-known instance of Differential Object Marking (DOM; Torrego, 1998; Leonetti, 2004). Recent studies have documented the loss and/or incomplete acquisition of several grammatical features in Spanish heritage speakers (Silva-Corvalan, 1994; Montrul, 2002), including DOM (Montrul, 2004a). This study assesses the extent of incomplete knowledge of DOM in Spanish heritage speakers raised in the United States by comparing it with knowledge of DOM in fully competent native speakers. Experiment 1 examined whether omission of a affected grammatical competence, as measured by the linguistic behavior of 67 heritage speakers and 22 monolingual speakers in an oral production task and in a written acceptability judgment task. Experiment 2 followed up on the results of the acceptability judgment task with 13 monolingual speakers and 69 heritage speakers, and examined whether problems with DOM generalize to other instances of structural and inherent dative case, including ditransitive verbs and gustar-type psychological verbs. Results of the two experiments confirmed that heritage speakers' recognition and production of DOM is probabilistic, even for speakers with advanced proficiency in Spanish. This suggests that many heritage speakers' grammars may not actually instantiate inherent case. We argue that language loss under reduced input conditions in childhood is, in this case, like "going back to basics": it leads to simplification of the grammar by letting go of the non-core options, while retaining the core functional structure.