Does bilingualism facilitate the development of cognitive abilities, and if so, how? According to the level of bilingualism hypothesis (Diaz, 1983), only in the early stages of second language acquisition does bilingualism foster cognitive ability. This hypothesis was tested on a sample of 3rd and 4th grade Mexican Spanish-English bilinguals with low English proficiency. Knowledge of Spanish and English in phonology, vocabulary, and syntax was measured. Nonverbal general intelligence was assessed with the Raven Colored Progressive Matrices, and verbal intelligence was assessed with a subtest of the Woodcock Language Proficiency Battery. No relationship was found between degree of bilingualism and nonverbal intelligence, contrary to the level of bilingualism hypothesis. The results suggest that the effects of bilingualism on cognitive development are not solely dependent on the level of second language proficiency.