The purpose of this study was to examine the impact of at-risk student mobility on academic achievement in an urban elementary school. Math and reading scores from the Texas Assessment of Academic Skills (TAAS) of 172 third, fourth, and fifth grade students from an urban school district in South Central Texas were examined to determine whether student mobility had a differential effect on student achievement. Results indicated there were no significant differences in reading achievement among mobile and nonmobile students. However, nonmobile students outperformed mobile students in 5 out of the 13 domains in mathematics. The findings are discussed within a framework explaining why mobile and nonmobile students achieve similarly in reading, but differentially in mathematics. Implications from this study provide direction to school leaders on the importance of accommodating the learning needs of at-risk mobile students who enter new environments. (Contains 5 tables.)