This study investigates the acquisition of a non-null-subject language (English) by speakers of two different null-subject languages (Spanish and Chinese) in light of recent research in theoretical syntax which shows that different syntactic mechanisms are at work in the expression of null subjects in these two languages. While null subjects in Spanish are manifestations of pro, in Chinese they may be pro or a null topic, the latter licensed by a topic chain (Huang, 1984; 1989). Topic chains have been shown to license null topics in other languages (Roeper and Weissenborn, 1990; Weissenborn, 1992) but cannot do so across a lexically filled CP. We propose that this difference in licensing mechanism ought to affect the acquisition of English by Chinese and Spanish speakers. Data from an elicited imitation task show that Chinese speakers significantly outperform Spanish speakers in disallowing null subjects in English. This can be attributed to the Chinese speakers' reaction to the presence of lexically filled CPs in English, which prevents them from licensing null subjects via topic chains. Spanish speakers, on the other hand, because they license null subjects via Agr (Agreement), are unaffected by lexically filled CPs in English and continue to allow null subjects. Interestingly, however, Spanish speakers at higher levels of proficiency reject null subjects in certain contexts, suggesting that reanalysis of this feature of English can and may occur. © Arnold 1999.