This study investigated the effects of multigrade classes on student achievement in the Mesa Public Schools in Reading and Mathematics. Student achievement in differing abilities groups and levels was also analyzed. The District formed multigrade classes from adjacent grade levels to reduce class loads and number of teachers. Students retained their grade-level assignments and maintained their grade-specific curricula. The sample included 3,360 third through sixth graders in three groups: multigrade classes; single-grade classes from schools with multigrade classes; and single-grade classes from schools without multigrade classes. The 1981 and 1982 California Achievement Tests results were analyzed. Results indicated multigrade classes have no detrimental effects on reading or mathematics achievement with one exception: the mathematics achievement of average students. Using multigrade classes with only average/high students and identifying appropriate instructional techniques was recommended. An eight-page bibliography is included. (BS)