This study examined the theory asserting that partitioning a unit is basic in developing understanding of the different rational number constructs. A fraction sequence was developed in which early experiences with partitioning units were provided. An alternative fraction sequence was designed to include initial activities with pattern blocks in which fractional parts of a region are covered by blocks instead of drawing lines or splitting sets. Both fraction sequences were taught for 2 weeks in two fourth-grade classes of mostly language minority students from low social-economic status (SES) families. A repeated-measures design was adopted using a 40-item instrument assessing 8 different fraction topics. In addition, three students from each class were individually interviewed. Analysis of covariance did not indicate that the experimental fraction curriculum was superior than the alternative one while results from the videotaped interviews indicated that students' fraction knowledge was incomplete and unstable during the 4-week period. The interviews revealed students' strategies in dividing units and in using concrete materials which clearly influenced their fraction ideas. It is concluded that the findings support the theory which views partitioning a unit as critical in building rational number concepts. Contains 17 references. (Author/JRH)