Similar morphosyntactic development in both languages and low rates of language mixing (LM) attested in the corpus of a Spanish-Basque bilingual child rules out language dominance in the period studied (1;06–3;06). No preference for either language is observed with regard to lexical or functional elements in mixed phrases, restricted by the morphosyntactic rules pertaining to each lexical item in the corresponding language from the very beginning. This supports the view that mixed utterances are produced by one computational system operating with two lexicons, following MacSwan (2000), and that child mixing is not different from adult mixing as suggested by Cantone (2005). Finally, the slight increase observed for extra-sentential mixing in Basque contexts from age 3 onwards may be related to some kind of discursive development.