Co-teaching classrooms consist of general and special education teachers working together to benefit students with disabilities (SWDs). Many parents and teachers believe the content knowledge provided by general educators in the regular education setting, combined with the instructional service delivery model (SDM) expertise of special educators, will result in the greatest academic growth for SWDs. However, it is not known if SWDs served in one SDM progress at a faster rate than SWDs served in another SWD. Therefore, the focus of this study was to compare the reading and mathematics achievement of male and female middle school SDWs in two special education SDMs. A causal-comparative research design was used to analyze archival data from two consecutive years of the Georgia Milestones Assessments in Grades 6-8 for male and female SWDs with individual education plans who were taught reading and mathematics in either co-teaching classrooms or small-group resource rooms. Two two-way analyses of covariance were used to analyze the mean achievement scores of the inclusion and self-contained student groups. The independent variables were type of service delivery model (co-teaching classroom or small-group resource room) and gender. The dependent variables were mathematics and reading scores. A covariate was used to control for previous mathematics and reading achievement. An analysis of the data indicated that for both reading and mathematics, SWDs in co-teaching service delivery models scored significantly higher than their peers did in small-group resource classrooms. These findings support the supposition that co-teaching is an effective instructional model for middle school students with disabilities. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]