Previous research indicates that developmental contexts influence language development. These developmental contexts include bilingualism, which is the focus of the current paper. This study analyses the influence that the acquisition of a language, Catalan, has when the simultaneous acquisition of another similar language, Spanish, occurs. This influence is examined according to degree of exposure to the languages. The parents of 801 children completed the Catalan version of the MacArthur-Bates CDI-II questionnaire, which assesses the acquisition of vocabulary, morphology and grammatical complexity. Subjects were divided into three groups according to age and linguistic context (Catalan monolingual, medium bilingual and familiar bilingual). The results do not reveal substantial differences according to linguistic context. However, children aged 26--30 months with a medium bilingual profile show a higher level of vocabulary as compared to the other groups. The results are discussed in relation to the sociolinguistic context, caregiver diversity, typological distance between the languages and developmental stages of language acquisition. [Translation from Spanish by Jennifer Martin.]