We used event-related brain potentials (ERPs) to investigate the contributions of explicit and implicit processes during second language (L2) sentence comprehension. We used a L2 grammaticality judgment task (GJT) to test 20 native English speakers enrolled in the first four semesters of Spanish while recording both accuracy and ERP data. Because end-of-sentence grammaticality judgments are open to conscious inspection, we reasoned that they can be influenced by strategic processes that reflect on formal rules and therefore reflect primarily offline explicit processing. On the other hand, because ERPs are a direct reflection of online processing, they reflect automatic, nonreflective, implicit responses to stimuli (Osterhout, Bersick, & McLaughlin, 1997; Rugg et al., 1998; Tachibana et al., 1999). We used a version of the GJT adapted for the ERP environment. The experimental sentences varied the form of three different syntactic constructions: (a) tense-marking, which is formed similarly in the first language (L1) and the L2; (b) determiner number agreement, which is formed differently in the L1 and the L2; and (c) determiner gender agreement, which is unique to the L2. We examined ERP responses during a time period between 500 and 900 ms following the onset of the critical (violation or matched control) word in the sentence because extensive past research has shown that grammatical violations elicit a positive-going deflection in the ERP waveform during this period (e.g., the "P600"; Osterhout & Holcomb, 1992). We found that learners were sensitive (i.e., showed different brain responses to grammatical and ungrammatical sentences) to violations in L2 for constructions that are formed similarly in the L1 and the L2, but were not sensitive to violations for constructions that differ in the L1 and the L2. Critically, a robust grammaticality effect was found in the ERP data for the construction that was unique to the L2, suggesting that the learners were implicitly sensitive to these violations. Judgment accuracy was near chance for all constructions. These findings suggest that learners are able to implicitly process some aspects of L2 syntax even in early stages of learning but that this knowledge depends on the similarity between the L1 and the L2. Furthermore, there is a divergence between explicit and implicit measures of L2 learning, which might be due to the behavioral task demands (e.g., McLaughlin, Osterhout, & Kim, 2004). We conclude that comparing ERP and behavioral data could provide a sensitive method for measuring implicit processing. (Contains 1 footnote.)