In this study we compared the effects of smallgroup constructivist and explicit mathematics instruction in basic multiplication on low-achieving students' performance and motivation. A total of 265 students (aged 8-11 years) from 13 general and 11 special elementary schools for students with learning and/or behavior disorders participated in the study. The experimental groups received 30 minutes of constructivist or explicit instruction in groups of 5 students twice weekly for 5 months. Pre- and posttests were conducted to compare the effects on students' automaticity, problem-solving, strategy use, and motivation to the performance of a control group who followed the regular curriculum. Results showed that the math performance of students in the explicit instruction condition improved significantly more than that of students in the constructivist condition, and the performance of students in both experimental conditions improved significantly more than that of students in the control condition. Only a few effects on motivation were found. We therefore concluded that recent reforms in mathematics instruction requiring students to construct their own knowledge may not be effective for low-achieving students.