In this quasi-experimental pretest-posttest study we examined the effects of differentiated instruction (DI) in within-class ability groupings of 246 Montenegrin fourth-graders and their ability to solve algebraic equations. We assessed 2 parallel student groups at equal achievement levels to compare DI, in which teaching and work modes were adapted to students' grouping according to previous achievement and pretest scores, and traditional whole-class instruction. Pretest-posttest evaluations were administered to both groups, and observation indicators were evaluated to assess the level and type of student activities, engagement, and individualisation. Students in the homogeneous DI experimental groups with tailored instructions were significantly more successful at solving algebraic tasks than their peers in the traditional whole-class instruction control group. DI improved students' results, but teachers required specific training and significantly more preparation time.