Though proficiency with computation is highly emphasized in national mathematics standards, students with mathematics difficulty (MD) continue to struggle with computation. To learn more about the differences in computation error patterns between typically achieving students and students with MD, we assessed 478 3rd-grade students on a measure of mathematics computation. Results indicated that using the wrong operation was the most common identifiable error for all students. Students with MD had similar accuracy rates for item categories (e.g., addition items) compared to typically achieving students, but students with MD consistently had more variability in incorrect item responses. This study has implications for efficacious computation instruction for students in the elementary grades. [At the time of submission to ERIC, this article was in press with "Assessment for Effective Intervention."]