The present study examines how second language learners (L2) assign the thematic roles of agent/patient in Spanish passive sentences with "ser" (often referred to as the true passive) when it is their initial exposure to this structure. The target sentences were preceded by a contextual sentence. After hearing the two sentences, participants were asked to identify the agent of the passive sentence. All the sentences were reversible, and other linguistic variables were manipulated. Variables that have been shown in other studies on other linguistic structures to promote correct thematic role assignment include: the number (singular/ plural) of the agents and patients; the gender (masculine to feminine and feminine to masculine) of the agents and patients; and co-indexing of the grammatical subjects (+/-) in the context and target sentences. The quantitative analyses reveal that these native speakers of English, to varying degrees, assign thematic roles in passive sentences incorrectly. Furthermore, these learners show no sensitivity to number, gender, and co-referencing of grammatical subjects. None of the paired conditions promoted greater accuracy in assigning thematic roles. The results are discussed in terms of the First Noun Principle (VanPatten 2007) and the Canonical Alignment Hypothesis (Hyams et al. 2006).