Since 2015, increased numbers of newly immigrated schoolchildren in Europe have resulted in divergent, often ad hoc measures to provide for their education. Because the basis of classroom learning is information found in written texts, the development of grade-level reading skills is of central importance. However, little is known about immigrant students’ reading skills at and following transition, and no data is available for Germany, where the study was conducted. We report the results of a longitudinal study in which migrant students’ (N = 136) reading subskills after transition into mainstream were investigated at three points over the course of 2 years and compared to cohort performance (N = 517) in grades 7 through 9. Results showed that immigrant students performed significantly below mainstream students on all measures for all data points, with little evidence that they are beginning to close the gap even after several years in mainstream.