This article analyzes the interfering effect of the second language (L2) on the first language (L1) in native Spanish speakers living in the United States. We examined 3 linguistic aspects: (a) syntactic comprehension, (b) verbal memory, and (c) calculation abilities. We carried out 2 different studies. In the 1st study, we studied syntactic understanding in 50 Spanish-English bilinguals. For all participants, L1 was Spanish and L2 was English, and all learned English early in life and had attended English schools. Results for the Spanish Syntactic Comprehension Test (Marcos & Ostrosky, 1995) were compared with the normative results obtained with 40 Spanish monolingual participants. We observed that the closer to the English syntax the sentences were, the easier it was for the participants to understand them. Participants who had been exposed to English between the ages of 5 and 12 outperformed participants exposed to English before 5 years of age. Language preference correlated with syntactic comprehension. Women outperformed men. In the 2nd study, verbal memory and calculation abilities were examined in L1 and L2 in a group of 85 Spanish-English bilinguals. Parallel versions of the different tests were administered in Spanish and English. The results indicated some significant differences between the 2 languages in several verbal learning and calculation ability subtests. Most of the verbal memory subtests were better performed in L1. Scores on tasks measuring speed and calculation accuracy were higher in the participant's native language. Best spoken language proved to be a significant variable in some verbal memory subtests performed in English but not in Spanish. We analyze implications of bilingualism in neuropsychological testing. We also present some suggestions to minimize the bilingualism effect.