Co-teaching is a promising practice for educating students with disabilities in regular education classes. However, teachers often report being given co-teaching assignments without requisite training. Without adequate preparation, many teachers have difficulty conceptualizing co-teaching as a model and working collaboratively as teaching partners, often creating a division of labor that relegates special educators to a "helper" role in the classroom. This experimental study utilized a randomized pretest-posttest control group design to study the effects of a professional development training package on the collaborative teaching performance of regular and special education teachers. Analysis of covariance showed that teachers who participated in professional development training on co-teaching had significantly higher posttest scores on a co-teaching performance assessment than those who did not participate in training.