Declines in students' achievement motivation across the transition to middle school may be explained by characteristics of both the academic and social environment of the new school. This study proposes that students' sense of belonging in middle school and their endorsement of social responsibility, relationship, and status goals in that setting should explain, in part, changes in their achievement goal orientations between 5th and 6th grades. Longitudinal survey data from 660 students indicated that, on average, endorsement of personal task goals declined, whereas endorsement of ability goals increased across the transition. Increases in task goal orientation were associated with perceiving both a task and an ability goal structure in 6th grade classes, along with sense of school belonging, and endorsing responsibility goals. Increases in ability goal orientation were associated positively with perceiving an ability goal structure in classes, with relationship and status goals, and negatively with school belonging. Copyright 1999 Academic Press.