Abstract The goal of this study is to shed light on how empirical data on the discourse constraints of null and overt third person subject pronouns in L1 and bilingual Spanish meet linguistic theory. A (semi)spontaneous production task was administered to 34 Moroccan Arabic (MA)/Spanish early sequential bilinguals and 30 L1 Spanish controls. All 3rd person subject positions were coded: (1) morphosyntactic form (null pronoun vs. overt pronoun); (2) discourse function ([-Topic Shift] vs. [+Topic Shift]); (3) sentence relation (intrasentential vs. intersentential); (4) clause order within intrasentential contexts (main-subordinate vs. subordinate-main); and (5) access to the antecedent (clear vs. ambiguous antecedent). The results reveal general patterns of use in both L1 and bilingual Spanish: null pronouns express topic maintenance both in inter- and intrasentential contexts (both clause orders) and overt pronouns, especially in intersentential contexts, are generally used for topic change. However, additional analyses provide evidence that null pronouns in L1, but not in bilingual Spanish, are often used in change of reference contexts where the antecedent is not ambiguous. This reveals patterns that have gone unreported by most previous descriptive and theoretical studies. Finally, a higher use of ambiguous null pronouns is attested among bilingual speakers, which suggests a lower control of the mechanisms by which reference is established in discourse and supports, to some extent, the predictions derived from the Interface Hypothesis.