Background: The relation between academic self-concept and achievement has been examined in a large number of studies. The majority of these studies have found evidence for a reciprocal effects model. However, there is an ongoing debate on how students' achievement should be measured and whether the type of achievement indicator (grades, tests, teacher ratings) affects the causal pattern found in these studies. Aims: The study aims at clarifying how the types of achievement measures and the way they are modelled can affect the results of causal ordering studies. In that sense, the study will yield recommendations for researchers in this domain and also provide some direction for practitioners seeking ways to enhance their students' achievement and/or academic self-concept. Sample: Repeated measures of academic self-concept and achievement (standardized tests and teacher ratings) were examined in a sample of 1,753 students in Grades 7, 8, 10, and 12. Method: Structural equation modelling was used. Several models (with different types and numbers of achievement measures) were compared. Results: Only small differences were found between models using one or two indicators of achievement. All models generally supported the reciprocal effects model. However, the final model, wherein tests and teacher ratings were used as separate latent variables, showed different developmental patterns in the causal relation between academic self-concept and achievement. Conclusions: Researchers should interpret the results of causal ordering studies discerningly because the type of measure chosen as an indicator of achievement might affect the causal pattern between academic self-concept and achievement.