This paper examines the effects of roommates on students' academic outcomes exploiting the randomized roommate assignment system at a selective Chinese university. Unlike earlier studies that could not measure students' academic quality precisely, this paper makes two important improvements in measuring their English proficiency and overall academic aptitude. I find that randomly assigned roommates' average pre-treatment academic abilities are not significant determinants of students' academic performance across a number of different specifications. However, students are significantly influenced by roommates' decisions in terms of the number of elective course credits taken and the choice of major.