The acquisition by monolingual children of the knowledge of whether their language requires overt subjects is one of the most studied phenomena in the language acquisition literature, but this phenomenon has not received the same degree of attention in bilingual children. The present study contributes to closing this gap by focusing on subject use in an overt subject language, English, and a null subject language, Spanish, by a child acquiring these two languages from birth. The frequency of subject usage is determined at five MLUw stages between the ages of 1;5:8 and 2;8:9, and the paper examines discourse-pragmatic and processing factors that may account for the variable realization of subjects in each language. The analysis shows that the child realizes at a very early age that English requires overt subjects and Spanish does not. The results also indicate that in regard to subject use in Spanish, there does not exist an immature interface between grammar and the discourse-pragmatic domain. The child expresses subjects in contexts where monolingual speakers would also use them: contrastive and focal subjects are expressed, and coreferential subjects are not. With respect to the issue of interlinguistic influence, the study leaves no doubt that Spanish and English develop autonomously with respect to subject expression. The child seems to be matching the input he receives in English and Spanish without showing any effect of one language on the other.