Lexical phrases are both numerous and functionally important in written texts. Despite this, L2 learners often find their use problematic, typically overusing a limited number of well-known phrases, while at the same time lacking a diverse enough phrasal repertoire to employ lexical phrases in a native-like manner. While a number of studies have described learners' usage of lexical phrases, very few have explored how these phrases are acquired. In this paper we report on a longitudinal case study which followed a Chinese MA student over the course of an academic year. All of her written assignments (8 essays and a dissertation) were analyzed for lexical phrase use, and she was interviewed after each assignment was submitted. It was found that she learned 166 new lexical phrases during her studies, and that she improved in her degree of appropriate usage. She also gained confidence in using the phrases. She successfully drew upon both explicit and implicit sources for this improvement, particularly benefiting from her academic reading. However, she also tended to rely too heavily on a limited range of phrases, sometimes to the point where judges considered the usage non-nativelike. We discuss the relevance of these findings for our current knowledge of L2 writing. (Contains 4 figures and 7 tables.)