Over the past decade, there have been diverse theoretical perspectives and increasing empirical literature on bilingualism and specific language impairment (SLI), some of which highlighted the complex nature of accurately diagnosing SLI in bilingual populations. The goal of the current study is to enhance our understanding of morphosyntax development in an understudied bilingual population - Mandarin-English children who are growing up in an L2-dominant environment (English) in the United States. The study included a total of 55 bilingual Mandarin-English children between the ages of four and seven years, including 53 typically developing (TD) children and 2 children diagnosed with SLI. Using a newly developed screening test - the Bilingual English-Mandarin Oral Screener (BEMOS), we compared Mandarin performance in both TD and SLI children on 7 morphosyntax tasks which respectively measure passive -bei, possessive -de, prepositional phrases, noun classifiers, quantifier and scope, aspects (imperfective “-zai” and perfective “-le”), and sentence repetition. Our analysis of TD bilingual children revealed a trend towards a significant age effect in the total score and a near-significant effect in the preposition and the aspect sub-sections of the screener. When age was considered, perceived Mandarin proficiency by parents was associated with TD bilingual children’s performance. All students performed poorly on the classifier section, but our error analysis showed a predominant response pattern of imitation, suggesting bilingual children have growing sensitivity and are attentive to semantic similarity of nouns. Overgeneralized use of the general classifier “ge” was also observed in the errors. Both children with SLI scored lower overall compared to their age- and gender-matched TD peers, especially in the classifier and quantifier & scope sections. Reliable clinical markers were not identified due to the two SLI children’s distinct performance. Clinical implications and future research needs were also discussed. / text