Randomly assigned groups of fourth grade students were used to determine the effect of class size upon the learning of ten mathematical objectives. There were 20 groups of one, ten groups of two, four groups of five, and one group of 23 for each of three schools used in the study. Within schools, seven randomly assigned teachers presented the lesson and tested immediately after presentation. Students in each of the smaller class sizes displayed significantly greater attainment of the ten mathematical objectives than did those in classes with 23 students. Also, one-to-one instruction was significantly superior to one-to-five. (Author/JG)