Geographic mobility has a negative effect on a student's overall academic achievement. Data were drawn from Denver (Colorado) Public Schools (DPS), a multiethnic urban school system of over 60,000 students of whom 58,400 are enrolled in grades K through 12. Each year, students at all grade levels are administered the Iowa Tests of Basic Skills (ITBS) for the elementary grades and the Tests of Academic Progress (TAP) for high school grades. Composite ITBS or TAP scores served as the dependent variable for the analysis. Student mobility was defined by student enrollment patterns in the period from September 1985 through March 1987 when they were tested with the ITBS or TAP for the 1986-87 academic year. Three groups of continuing students and two groups of newly entered students were analyzed. Findings include the following: (1) the percentage of students classified as mobile decreases as grade level increases; (2) achievement levels of the more stable groups were consistently higher than those of students in the more mobile groups; and (3) attempts to control for student socioeconomic status did not alter the correlation between mobility and achievement. While economic and socio-cultural forces impel high mobility and instability among various groups, school administrators might discourage moves of a relatively small distance that place a child in a new assignment area. A list of references, five statistical tables, and two graphs are included. (FMW)