This study investigated whether crosslinguistic interference occurs in the domain of subject realization in Spanish in a bilingual acquisition context. We were also interested in exploring whether the source of the interference is due to child-internal crosslanguage contact between English and Spanish, as is commonly assumed, or due to the nature of the language input in a bilingual family, a factor which has not typically been considered in studies of crosslinguistic influence. The use of subjects in a null subject language like Spanish is a phenomenon linked to the pragmatics/syntax interface of the grammar, and thus, is a domain where crosslinguistic interference is predicted to be likely to occur in bilingual acquisition (Müller & Hulk, 2001). Using spontaneous language data available from CHILDES (www.childes.psy.cmu.edu), we examined the use of overt subjects in Spanish by two Spanish monolingual children (ages: 1;8–2;7 and 1;8–1;11) one Spanish–English bilingual child (age 1;9–2;6) and their parental interlocutors. We looked at the proportions of overt versus null subjects as well as the discourse-pragmatic contexts of overt subject use by the children in order to uncover bilingual/monolingual differences in the distributional properties and the functional determinants of subject realization. We also looked at identical variables in the speech of the children's parental interlocutors to investigate the potential influence of the input on the children's output. Our results suggest that the bilingual child showed patterns in her subject realizations in Spanish that could be interpreted as due to crosslinguistic effects from English; however, there is also evidence that these effects may have a source in the input, rather than resulting from internal crosslanguage contact. While our data do not permit us to distinguish conclusively between these two possible sources, they indicate that future research on crosslinguistic influence in bilingual acquisition should take input into account.