The authors compared the performance of students who received integration of number sense activities in instruction with students who received instruction using regular mathematics textbooks. Two classes of third-grade students (N = 60) were randomly assigned to experimental and control groups. Students in each group were given a pretest, post I test, and post II test, and were interviewed individually across 20 class periods of intervention. Results showed that the performance of the students in the experimental group was significantly higher than those in the control group. The interview data also showed that the students in the experimental group performed better than students in the control group in flexible and effective ways to use number sense. (Contains 3 figures and 5 tables.)