This article is part of a longitudinal study of American college students studying Mandarin in China. Its main data are drawn from monthly speaking events (four sessions with each student) conducted in Chinese with each of 29 participants in a study-abroad program in China. The study yielded these results: (a) Students made significant progress in their fluency development, especially during the first month; (b) speaking Chinese both inside and outside of class helped fluency; (c) students who consistently spoke Chinese in class and out of class made more progress than those who did not; and (d) time on task, the amount of time that students spent using Chinese, was the most important factor in determining fluency development. The quantitative analysis that is the focus of this article is supplemented by qualitative data from individual students, reflecting the position that only a combination of quantitative and qualitative data can reveal a complete picture of the nature of language acquisition during study abroad. The article concludes with implications for the design and conduct of programs in China.