The purpose of the study was to investigate the effect of ability grouping classes on 7th grade students’ academic achievement on the unit “Ya Basınç Olmasaydı?” in Science and Technology Education. The study was conducted in a public school where ability grouping classes was practiced in the second semester of 2006-2007 academic year in Aydın. Total 84 students among the 7th grader in the ability grouping classes of the selected school constituted the sample of the study. “Science Education achievement test” developed by the researchers was used for data collection. Co-variance analysis was performed for analyzing the data gathered in the study. Significant differences were found among the ability grouping classes with regard to achievement test scores (p=.000). The difference was observed between high-medium grouping classes, high-low grouping classes and medium-low grouping classes. While significant difference was not found among achievement scores of the female students in ability grouping classes (p=.220), this was reverse for the male students (significant difference was found among the male students with regard to achievement test scores).