This paper examines the emergence of perfective aspect in Cantonese-English bilingual children from the perspective of contact-induced grammaticalization, focusing on the novel use of already. Although the adverbial already seems to serve a function similar to that of the Cantonese perfective marker zo2 in the bilingual children, other model constructions suggest that the function of already may combine those of several Cantonese particles such as the sentence-final particle laa3. The results suggest that in contact-induced grammaticalization, it is possible to develop a new category in the replica language based on multiple different but related categories in the model language. Adopting an evolutionary approach to language transmission (Mufwene, 2001), we discuss why grammaticalization in the Cantonese-English bilingual children does not seem to involve coevolution of form and meaning, why the grammaticalization phenomena in the bilingual children are only transient, and how the study of bilingual acquisition can contribute to contact linguistics.