With Chinese native-speaker data as the baseline, this study investigates the use of the morphosyntactic particle DE by learners of Chinese as a second language. The general patterns are as follows: (a) DE tends to be deleted more in informal speech than in formal settings; (b) higher proficiency and longer residence in China--more interactions with native speakers--promote DE deletion; and (c) females tend to adopt more formal language style and use DE more than males. The study also found that teachers and textbooks use DE much more often than native speakers. Learners' patterns of DE use closely follow those of their teachers and textbooks, suggesting the necessity of explicit instruction in sociolinguistic variants in L2 classrooms.