As education continues to progress schools are constantly seeking innovative ways to cultivate and enhance achievement for all students. As a result many public schools are pushing toward the inclusion model. This model includes co-taught instruction to meet the many needs of special education students. This research study was implemented to investigate the comparative effects of co-teaching versus solo-teaching on student's math achievement in elementary school. Study participants included two fourth grade classes in an elementary school, one with a regular education (solo-teaching) and the other with the same regular education teacher and a special education teacher for the co-taught class. The independent variable is the teaching arrangement (co taught class vs. a solo-taught class) as considered by the school system and the dependent variable is the math achievement as measured by Number Sense, Multiplication, and Division pre and post test units. Comparison of student math achievement between co-teaching and solo-teaching showed that solo teaching was more effective than co-teaching on student's achievement in Multiplication, co-teaching was more effective on student achievement in the Number sense unit than solo-teaching, and that no statistical difference was shown between co-teaching and solo teaching in their effect on student learning in the Division unit. It is concluded that both solo teaching and co-teaching were beneficial to the two different groups of students within their various learning environments. Further experimental research is needed. (Contains 5 tables.)